CHAPTER 22 OBJECTIVES

1

• CHAPTER 22 OBJECTIVES
• Explain the purpose and function of the ECT and
  IAT temperature sensors.
• Describe how to test temperature sensors.
• Discuss how throttle position sensors work.
• List the methods that can be used to test TP
  sensors.
• Discuss how MAP sensors work.
• List how the operation of the MAP sensor affects
  vehicle operation.
• Discuss how MAF sensors work.
• Discuss how O2S sensors work.
• List how the operation of the O2S sensor affects
  vehicle operation.

2

• Engine Coolant Temperature Sensors
• The ECT sensor is also used as an important input
  for the following
• Idle air control (IAC) position
• Oxygen sensor closed loop times
• Canister purge on/off times
• Idle speed
• Engine coolant temperature sensors are
  constructed of a semiconductor material that
  decreases in resistance as the temperature of the
  sensor increases.

3

• Engine Coolant Temperature Sensors (continued)
• Coolant sensors have very high resistance when
  the coolant is cold and low resistance when the
  coolant is hot.
• This is referred to as having a negative
  temperature coefficient (NTC).

4

• Testing The Engine Coolant Temperature Sensor
• Testing the ECT using a multimeter
• Both the resistance (in ohms) and the voltage
  drop across the sensor can be measured and
  compared with specifications.

5

• Testing The Engine Coolant Temperature Sensor
  (continued)
• General Motors ECT sensor

6

• Testing The Engine Coolant Temperature Sensor
  (continued)
• Always consult the manufacturers recommended
  procedures for checking this wiring.
• Normal operating temperature varies with vehicle
  make and model.
• Some vehicles are equipped with a thermostat with
  an opening temperature of 180 F (82 C), whereas
  other vehicles use a thermostat that is 195 F
  (90 C) or higher.
• Before replacing the ECT sensor, be sure that the
  engine is operating at the temperature specified
  by the manufacturer.
• Most manufacturers recommend checking the ECT
  sensor after the cooling fan has cycled twice,
  indicating a fully warmed engine.
• Testing the ECT sensor using a scan tool
• Comparing the temperature of the engine coolant
  as displayed on a scan tool with the actual
  temperature of the engine is an excellent method
  to test an engine coolant temperature sensor.
• Record the scan tool temperature of the coolant
  (ECT).
• Measure the actual temperature of the coolant
  using an infrared pyrometer or contact-type
  temperature probe.

7

• Intake Air Temperature Sensor
• The intake air temperature (IAT) sensor is a
  negative temperature coefficient (NTC) thermistor
  that decreases in resistance as the temperature
  of the sensor increases.
• The IAT sensor can be located in one of the
  following locations
• In the air cleaner housing
• In the air duct between the air filler and the
  throttle body
• Built into the mass air flow (MAF) or air flow
  sensor
• Screwed into the intake manifold where it senses
  the temperature of the air entering the cylinders
• The IAT sensor information is used for fuel
  control (adding or substituting fuel) and spark
  timing, depending on the temperature of incoming
  air.
• If the air temperature is cold, the PCM will
  modify the program amount of fuel delivery and
  add fuel.
• If the air temperature is hot, the PCM will
  subtract the calculated amount of fuel.

8

• Intake Air Temperature Sensor (continued)
• Spark timing is also changed, depending on the
  temperature of the air entering the engine. The
  timing is advanced if the temperature is cold and
  retarded from the base programmed timing if the
  temperature is hot.
• Cold air is more dense and contains more oxygen
  and therefore requires a richer mixture to
  achieve the proper air-fuel mixture. Air at 32
  F (0 C) is 14 denser than air at 100 F (38
  C).
• Hot air is less dense and contains less oxygen
  and therefore requires a leaner mixture to
  achieve the proper air-fuel mixture.
• The IAT sensor is a low-authority sensor and is
  used by the computer to modify the amount of fuel
  and ignition timing as determined by the engine
  coolant temperature sensor.
• The IAT sensor is used by the PCM as a backup in
  the event that the ECT sensor is determined to be
  inoperative.

9

• Throttle Position Sensors
• Engines use a throttle position (TP) sensor to
  signal to the computer the position of the
  throttle.

10

• Throttle Position Sensors (continued)
• The TP sensor consists of a potentiometer
  variable resistor.
• A potentiometer is a variable-resistance sensor
  with three terminals.
• One end of the resistor receives reference
  voltage, while the other end is grounded.
• The third terminal is attached to a moveable
  contact that slides across the resistor to vary
  its resistance.
• Depending on whether the contact is near the
  supply end or the ground end of the resistor,
  return voltage is high or low.

11

• Throttle Position Sensors (continued)
• A typical sensor has three wires
• A 5-volt reference feed wire from the computer
• A ground wire back to the computer
• A voltage signal wire back to the computer as
  the throttle is opened, the voltage to the
  computer changes
• Normal throttle position voltage on most vehicles
  is about 0.5 volt at idle (closed throttle) and
  4.5 volts at wide-open throttle (WOT).

12

• TP Sensor Computer Input Functions
• The computer senses this change in throttle
  position and changes the fuel mixture and
  ignition timing.
• The throttle position (TP) sensor used on
  fuel-injected vehicles acts as an electronic
  accelerator pump. This means that the computer
  will pulse additional fuel from the injectors
  when the throttle is depressed.
• The PCM supplies the TP sensor with a regulated
  voltage that ranges from 4.8 to 5.1 volts. This
  reference voltage is usually referred to as a
  5-volt reference or Vref. The TP output signal
  is an input to the PCM and the TP sensor ground
  also flows through the PCM.

13

• Ford Throttle Position (TP) Sensor Chart
• NOTE Generally, any reading higher than 80
  represents wide-open throttle to the computer.

14

• PCM Uses For The TP Sensor
• Clear Flood Mode
• If the throttle is depressed to the floor engine
  cranking, the PCM will either greatly reduce or
  entirely eliminate any fuel injector pulses to
  aid in cleaning a flooded engine.
• Torque Converter Clutch Engagement and Release
• The torque converter clutch will be released if
  the PCM detects rapid acceleration to help the
  transmission deliver maximum torque to the drive
  wheels.
• Rationality Testing for MAP and MAF Sensors
• As part of the rationality tests for the MAP
  and/or MAF sensor, the TP sensor signal is
  compared to the reading from other sensors to
  determine if they match.
• For example, if the throttle position sensor is
  showing wide open throttle (WOT), the MAP and/or
  MAF reading should also indicate that this engine
  is under a heavy load.

15

• PCM Uses For The TP Sensor (continued)
• Automatic Transmission Shift Points
• The shift points are delayed if the throttle is
  opened wide to allow the engine speed to
  increase, thereby producing more power and to aid
  in the acceleration of the vehicle.
• Target Idle Speed (Idle Control Strategy)
• When the TP sensor voltage is at idle, the PCM
  then controls idle speed using the idle air
  control (IAC) and/or spark timing variation, to
  maintain the commanded idle speed.
• Air-Conditioning Compressor Operation
• The TP sensor is also used as an input sensor for
  traction control and air-conditioning compressor
  operation.
• If the PCM detects that the throttle is at or
  close to wide open, the air-conditioning
  compressor is disengaged.
• Backs up Other Sensors
• The TP sensor is used as a backup to the MAP
  sensor and/or MAF in the even the PCM detects
  that one or both are not functioning correctly.
• The PCM then calculates fuel needs and spark
  timing based on the engine speed (RPM) and
  throttle position.

16

• Testing The Throttle Position Sensor
• A TP sensor can be tested using one or more of
  the following tools
• A digital voltmeter with three test leads
  connected in series between the sensor and wiring
  harness connector or back probing using T-pins.
• A scan tool or a specific tool recommended by the
  vehicle manufacturer.
• An oscilloscope.
• The procedure for testing the sensor using a
  digital multimeter is as follows
• Turn the ignition switch on (engine off)).
• Measure the voltage between the signal wire and
  ground (reference low) wire. The voltage should
  be about 0.5 volt.

17
Chapter 22
Computer Sensors
TP Sensor Diagnosis Photo Sequence
Diagnosis and Troubleshooting of Automotive
Electrical, Electronic and Computer Systems
18

• Testing The Throttle Position Sensor (continued)
• With the engine still not running (but with the
  ignition still on), slowly increase the throttle
  opening. The voltage signal from the TP sensor
  should also increase. Look for any dead spots
  or open circuit readings as the throttle is
  increased to the wide-open position.

19
Testing The Throttle Position Sensor
(continued) Use the accelerator pedal to depress
the throttle because this applies the same force
on the TP sensor as the drive does during normal
driving. Moving the throttle by hand under the
hood may not accurately test the TP sensor.
20

• Manifold Absolute Pressure (MAP) Sensors
• The manifold absolute pressure (MAP) sensor is
  used by the engine computer to sense engine load.

21

• Manifold Absolute Pressure (MAP) Sensors
  (continued)
• The relationship among barometer pressure, engine
  vacuum, and MAP sensor voltage includes
• Absolute pressure is equal to barometric pressure
  minus intake manifold vacuum.
• A decrease in minimum vacuum means an increase in
  manifold pressure.

22

• Manifold Absolute Pressure (MAP) Sensors
  (continued)
• The MAP sensor compares manifold vacuum to a
  perfect vacuum.
• Barometric pressure minus MAP sensor readings
  equals intake manifold vacuum. Normal engine
  vacuum is 17 21 in. Hg.
• Supercharged and turbocharged engines require a
  MAP sensor that is calibrated for pressures above
  atmospheric, as well as for vacuum.

23

• Silicon-Diaphragm Strain Gauge MAP Sensor
• This is the most commonly used design for a MAP
  sensor and the output is an analog variable
  voltage.
• One side of a silicon wafer is exposed to engine
  vacuum and the other side is exposed to a perfect
  vacuum.
• There are four resistors attached to the silicon
  wafer which changes in resistance when strain is
  applied to the wafer.

24
Silicon-Diaphragm Strain Gauge MAP Sensor
(continued)
25

• Capacitor Capsule MAP Sensor
• A capacitor-capsule is a type of MAP sensor used
  by Ford and it uses two ceramic (alumina) plates
  with an insulating washer spacer in the center to
  create a capacitor.
• Changes in engine vacuum cause the plates to
  deflect, which changes the capacitance.

26

• Capacitor Capsule MAP Sensor (continued)
• The electronics in the sensor then generates a
  varying digital frequency output signal, which is
  proportional to the engine vacuum.

27
Capacitor Capsule MAP Sensor (continued)
28
Capacitor Capsule MAP Sensor (continued)
29

• PCM Uses Of The MAP Sensor
• The PCM uses the MAP sensor to determine the
  following
• The load on the engine.
• Altitude, fuel, and spark control calculations.
• EGR system operation.
• Detect deceleration (vacuum increases).

30

• PCM Uses Of The MAP Sensor (continued)
• Monitor engine condition.
• Load detection for returnless-type fuel
  injection.
• Altitude and MAP sensor values.
• Barometric pressure and altitude are inversely
  related
• As altitude increase barometric pressure
  decreases
• As altitude decrease barometric pressure
  increases

31

• PCM Uses Of The MAP Sensor (continued)
• Altitude and MAP Sensor Values

32

• Testing The MAP Sensor Using A DMM
• Four different types of test instruments can be
  used to test a pressure sensor
• A digital voltmeter with three test leads
  connected in series between the sensor and the
  wiring harness connector
• A scope connected to the sensor output, power,
  and ground

33

• Testing The MAP Sensor Using A DMM (continued)
• A scan tool or a specified tool recommended by
  the vehicle manufacturer
• A breakout box connected in series between the
  computer and the wiring harness connection(s). A
  typical breakout box includes test points at
  which pressure sensor values can be measured with
  a digital voltmeter (or frequency counter, if a
  frequency-type MAP sensor is being tested)

34

• Testing The MAP Sensor Using A DMM (continued)
• Most pressure sensors use three wires
• A 5-volt wire from the computer
• A variable-signal wire back to the computer
• A ground or reference low wire

35

• Testing The MAP Sensor Using A DMM (continued)
• The procedure for testing the sensor is as
  follows
• Turn the ignition on (engine off)
• Measure the voltage (or frequency) of the sensor
  output
• Using a hand-operated vacuum pump (or other
  variable vacuum source), apply vacuum to the
  sensor

36

• Testing The MAP Sensor Using A DMM (continued)
• A good pressure sensor should change voltage (or
  frequency) in relation to the applied vacuum.
• If the signal does not change or the values are
  out of range according to the manufacturers
  specifications, the sensor must be replaced.

37

• Airflow Sensors
• The vane airflow sensor used in Bosch L-Jetronic,
  Ford, and most Japanese electronic port
  fuel-injection systems is a moveable vane
  connected to a laser-calibrated potentiometer.
• The vane is mounted on a pivot pin and is
  deflected by intake airflow proportionate to air
  velocity.
• As the vane moves, it also moves the
  potentiometer.

38

• Airflow Sensors (continued)
• There is a special dampening chamber built into
  the VAF to smooth out vane pulsations which would
  be created by intake manifold air-pressure
  fluctuations caused by the vane opening and
  closing.
• Many vane airflow sensors include a switch to
  energize the electric fuel pump.
• This is a safety feature that prevents the
  operation of the fuel pump if the engine stalls.

39

• Mass Airflow Sensor
• Hot Film Sensor
• The hot film sensor uses a temperature-sensing
  resistor (thermistor) to measure the temperature
  of the incoming air.
• Through the electronics within the sensor, a
  conductive film is kept at a temperature 70 C
  above the temperature of the incoming air.

40

• Mass Airflow Sensor
• Hot Film Sensor (continued)
• Because the amount and density of the air both
  tend to contribute to the cooling effect as the
  air passes through the sensor, this type of
  sensor can actually produce an output based on
  the mass of the airflow.
• Mass equals volume times density.
• Therefore, a mass airflow sensor is designed to
  measure the mass, not the volume of the air
  entering the engine.

41

• Mass Airflow Sensor
• Hot Film Sensor (continued)
• Most of these types of sensors are referred to as
  mass airflow (MAF) sensors because unlike the air
  vane sensor, the MAF sensor takes into account
  relative humidity, altitude, and temperature of
  the air.
• The denser the air, the greater the cooling
  affect on the hot film sensor and the greater the
  amount of fuel required for proper combustion.

42

• Mass Airflow Sensor (continued)
• Hot Wire Sensor
• The hot wire sensor is similar to the hot film
  type, but uses a hot wire to sense the mass
  airflow instead of the hot film.
• Like the hot film sensor, the hot wire sensor
  uses a temperature-sensing resistor (thermistor)
  to measure the temperature of the air entering
  the sensor.

43

• Mass Airflow Sensor (continued)
• Hot Wire Sensor (continued)
• The electronic circuitry within the sensor keeps
  the temperature of the wire at 70 C above the
  temperature of the incoming air.
• The operating principle can be summarized as
  follows
• More intake air volume cooler sensor, more
  current.
• Less intake air volume warmer sensor, less
  current.
• The computer constantly monitors the change in
  current and translates it into voltage signals
  that is used to determine injector pulse width.

44

• Mass Airflow Sensor (continued)
• Burn-off circuit
• Some MAF sensors use a burn-off circuit to keep
  the sensing wire clean of dust and dirt.
• A high current is passed through the sensing wire
  for a short time, but long enough to cause the
  wire to glow due to the heat.
• The burn-off circuit is turned on when the
  ignition switch is switched off after the engine
  has been operating long enough to achieve normal
  operating temperature.

45

• PC Uses For Airflow Sensors
• The PCM uses the information from the airflow
  sensor for the following purposes
• Airflow sensors are used mostly to determine the
  amount of fuel needed and base pulse-width
  numbers. The greater the mass of the incoming
  air, the longer the injectors are pulsed on.
• Airflow sensors back up the TP sensor in the
  event of a loss of signal or an inaccurate
  throttle position sensor signal. If the MAF
  sensor fails, then the PCM will calculate the
  fuel delivery needs of the engine based on
  throttle position and engine speed (RPM).

46

• Testing Mass Airflow Sensors
• Start the testing of a MAF sensor by performing a
  thorough visual inspection.
• Check the electrical connector for
• Corrosion
• Terminals that are bent or pushed out of the
  plastic container
• Frayed wiring

47

• MAF Sensor Output Test
• A digital multimeter can also be used to check
  the MAF sensor.
• See the chart that shows the voltage output
  compared with the grams per second of airflow
  through the sensor.
• Normal airflow is 3 to 7 grams per second.
• MAF sensor grams per second/voltage chart

48

• Tap Test
• With the engine running at idle speed, gently tap
  the MAF sensor with the fingers of an open hand.
• If the engine stumbles or stalls, the MAF sensor
  is defective.
• This test is commonly called the tap test.

49

• Digital Meter Test Of A MAF Sensor
• A digital multimeter can be used to measure the
  frequency (Hz) output of the sensor and compare
  the reading with specifications.
• The frequency output and engine speed in RPM can
  also be plotted on a graph to check to see if the
  frequency and RPM are proportional, resulting in
  a straight line on the graph.

50

• Contaminated Sensor Test
• Dirt, oil, silicon, or even spider webs can coat
  the sensing wire.
• Tests for a contaminated MAF sensor include
• At WOT, the grams per second, as read on a scan
  tool, should exceed 100.
• At WOT, the voltage, as read on a digital
  voltmeter, should exceed 4.
• At WOT, the frequency, as read on a meter or scan
  tool, should exceed 7 kHz.
• If the readings do not exceed these values, then
  the MAF sensor is contaminated.

51

• Oxygen Sensors
• In a zirconia oxygen sensor, the tip contains a
  thimble made of zirconium dioxide (ZrO2), an
  electrically conduction material capable of
  generating a small voltage in the presence of
  oxygen.
• Exhaust from the engine passes through the end of
  the sensor where the gases contact the outer side
  of the thimble.
• Atmospheric pressure enters through the other end
  of the sensor or thorough the wire of the sensor
  and contacts the inner side of the thimble.

52

• Oxygen Sensors (continued)
• The inner and outer surfaces of the thimble are
  plated with platinum.
• The inner surface becomes a negative electrode
  the outer surface is a positive electrode.
• The atmosphere contains a relatively constant 21
  percent of oxygen.

53

• Oxygen Sensors (continued)
• Rich exhaust gases contain little oxygen.
• Exhaust from a lean mixture contains more oxygen.
• Negatively charged oxygen ions are drawn to the
  thimble where they collect on both the inner and
  outer surfaces.

54

• Oxygen Sensors (continued)
• Because the oxygen present in the atmosphere
  exceeds that in the exhaust gases, the air side
  of the thimble draws more negative oxygen ions
  than the exhaust side.
• The difference between the two sides creates an
  electrical potential, or voltage.
• When the concentration of oxygen on the exhaust
  side of the thimble is low, a high voltage (0.60
  to 1.0 volts) is generated between the electrodes.

55

• Oxygen Sensors (continued)
• As the oxygen concentration on the exhaust side
  increases, the voltage generated drops low (0.00
  to 0.3 volts).

56

• Oxygen Sensors (continued)
• An O2S does not send a voltage signal until its
  tip reaches a temperature of about 572 F (300
  C).
• Also, O2 sensors provide their fastest response
  to mixture changes at about 1,472 F (800 C).
• When the engine starts and the O2S is cold, the
  computer runs the engine in the open loop mode,
  drawing on prerecorded data in the PROM for fuel
  control on a cold engine, or when O2S output is
  not within certain limits.

57

• Oxygen Sensors (continued)
• There are several different designs of oxygen
  sensors, including
• One-wire oxygen sensor
• The one wire of the one-wire oxygen sensor is the
  O2S signal wire.
• The ground for the O2S is through the shell and
  threads of the sensor and through the exhaust
  manifold.

58

• Oxygen Sensors (continued)
• Two-wire oxygen sensor
• The two-wire sensor has a signal wire and a
  ground wire for the O2S.
• Three-wire oxygen sensor
• The three-wire sensor design uses an electric
  resistance heater to help get the O2S up to
  temperature more quickly and to help keep the
  sensor at operating temperature even at idle
  speeds.
• The three wires include the O2S signal, the
  power, and ground for the heater.
• Four-wire oxygen sensor
• The four-wire sensor is a heated O2S (HO2S) that
  uses an O2S signal wire and signal ground.
• The other two wires are the power and ground for
  the heater.

59

• Zirconia Oxygen Sensors
• The greater the differences between the oxygen
  content between the inside and outside of the
  sensor.
• Rich mixture
• A rich mixture results in little oxygen in the
  exhaust stream.
• Compared to the outside air, this represents a
  large difference and the sensors create a
  relatively high voltage of about 1.0 volt (1000
  mV).

60

• Zirconia Oxygen Sensors (continued)
• Lean mixture
• A lean mixture leaves some oxygen in the exhaust
  stream and did not combine with the fuel.
• This left over oxygen reduces the difference
  between the oxygen content of the exhaust
  compared to the oxygen content of the outside
  air.
• As a result, the sensor voltage is low or almost
  zero volt.

61

• Zirconia Oxygen Sensors (continued)
• O2S voltage above 450 mV is produced by the
  sensor when the oxygen content in the exhaust is
  low. This is interpreted by the engine computer
  (PCM) as being a rich exhaust.
• O2S voltage below 450 mV is produced by the
  sensor when the oxygen content is high. This is
  interpreted by the engine computer (PCM) as being
  a lean exhaust.

62

• Titania Oxygen Sensor
• The titania (titanium dioxide) oxygen sensor does
  not produce a voltage but rather the presence of
  oxygen in the exhaust.
• All titania oxygen sensors use a four-terminal
  variable resistance unit with a heating element.
• A titania sensor samples exhaust air only and
  uses a reference voltage from the PCM.

63

• Titania Oxygen Sensor (continued)
• Titania oxide oxygen sensors use a 14-mm thread
  and are not interchangeable with zirconia oxygen
  sensor.
• One volt is applied to the sensor and the
  changing resistance of the titania oxygen sensor
  changes the voltage of the sensor circuit.
• As with a zirconia oxygen sensor, the voltage
  signal is above 450 mV when the exhaust is rich
  and low (below 450 mV) when the exhaust is lean.

64

• Wide-Band Oxygen Sensors
• A wide-band oxygen sensor, also called a lean
  air-fuel (LAF) ratio sensor or a linear air-fuel
  ratio sensor, allows engines to operate as lean
  as 231 and still maintain closed-loop operation.
• This type of sensor usually uses five wires.
• One power wire
• One ground wire for the electric heater
• Three sensor wires
• When the air-fuel mixture is perfectly balanced
  at 14.71, the sensor produces no output current.

65

• Wide-Band Oxygen Sensors (continued)
• When the air-fuel mixture is rich, the sensor
  produces a negative current ranging from zero to
  about 2 milliamps, which represents an air-fuel
  ratio of about 121.
• When the air-fuel ratio is lean, the sensor
  produces a positive current that ranges from zero
  to 1.5 milliamperes as the mixture gets leaner up
  to about 221.

66

• Closed Loop And Open Loop
• When the PCM alone is determining the amount of
  fuel needed, it is called open-loop operation.
• As soon as the oxygen sensor (O2S) is capable of
  supplying rich and lean signals, adjustments by
  the computer can be made to fine tune the correct
  air-fuel mixture.
• This checking and adjusting of the computer is
  called closed-loop operation.

67

• PCM Uses Of The Oxygen Sensor
• Fuel Control
• The upstream oxygen sensors are one of the main
  sensor(s) used for fuel control while operating
  in closed loop.
• Fuel Trim
• Diagnosis
• The oxygen sensors are used for diagnosis of
  other systems and components.
• For example, the exhaust gas recirculation (EGR)
  system is tested by the PCM by commanding the
  valve to open during the test.
• Some PCMs determine whether enough exhaust gas
  flows into the engine by looking at the oxygen
  sensor response (fuel trim numbers).

68

• PCM Uses Of The Oxygen Sensor (continued)
• Diagnosis (continued)
• The upstream and downstream oxygen sensors are
  also used to determine the efficiency of the
  catalytic converter.

69

• Testing An Oxygen Sensor Using A Digital
  Voltmeter
• The oxygen sensor can be checked for proper
  operation using a digital high-impedance
  voltmeter.
• With the engine off, connect the red lead of the
  meter to the oxygen sensor signal wire.

70

• Testing An Oxygen Sensor Using A Digital
  Voltmeter (continued)
• Start the engine and allow it to reach
  closed-loop operation.
• In closed-loop operation, the oxygen sensor
  voltage should be constantly changing as the fuel
  mixture is being controlled.
• The results should be interpreted as follows
• If the oxygen sensor fails to respond, and its
  voltage remains at about 450 millivolts, the
  sensor may be defective and require replacement.
  Before replacing the oxygen sensor, check the
  manufacturers recommended procedures.

71

• Testing An Oxygen Sensor Using A Digital
  Voltmeter (continued)
• If the oxygen sensor reads high all the time
  (above 550 millivolts), the fuel system could be
  supplying too rich a fuel mixture or the oxygen
  sensor may be contaminated.
• If the oxygen sensor voltage remains low (below
  350 millivolts), the fuel system could be
  supplying too lean a fuel mixture. Check for a
  vacuum leak or partially clogged fuel
  injector(s). Before replacing the oxygen sensor,
  check the manufacturers recommended procedures.

72
Testing The Oxygen Sensor Using The Min/Max Method
73

• Testing The Oxygen Sensor Using The Min/Max
  Method
• Min/max oxygen sensor test chart

74
Testing The Oxygen Sensor Using The Min/Max
Method (contd)
75

• Testing An Oxygen Sensor Using A Scan Tool
• A good oxygen sensor should be able to sense the
  oxygen content and change voltage outputs rapidly.

76

• Testing An Oxygen Sensor Using A Scan Tool
  (continued)
• To test an engine using a scan tool, follow these
  steps
• Connect the scan tool to the DLC and start the
  engine.
• Operate the engine at a fast idle (2500 RPM) for
  2 minutes to allow time for the oxygen sensor to
  warm to operating temperature.
• Observe the oxygen sensor activity on the scan
  tool to verify closed-loop operation. Select
  snapshot mode and hold the engine speed steady
  and start recording.

77

• Testing An Oxygen Sensor Using A Scan Tool
  (continued)
• Playback snapshot and place a mark beside each
  reach of oxygen sensor voltage for each frame of
  the snapshot.
• A good oxygen sensor and computer system should
  result in most snapshot values at both ends (0 to
  300 and 600 to 1000 mV).
• If most of the readings are in the middle, the
  oxygen sensor is not working correctly.

78

• Testing An Oxygen Sensor Using A Scope
• A scope can also be used to test an oxygen
  sensor.
• Connect the scope to the signal wire and ground
  for the sensor (if it is so equipped).

79

• Testing An Oxygen Sensor Using A Scope
  (continued)
• With the engine operating in closed loop, the
  voltage signal of the sensor should be constantly
  changing.

80

• Testing An Oxygen Sensor Using A Scope
  (continued)
• Check for rapid switching from rich to lean and
  lean to rich and change between once every 2
  seconds and 5 times per second (0.5 to 5.0 Hz).

81
Testing An Oxygen Sensor Using A Scope (continued)
82

• False O2S Readings
• False Lean
• False lean indications (low O2S readings) can be
  attributed to the following
• Ignition misfire. An ignition misfire due to a
  defective spark plug wire, fouled spark plug,
  etc., causes no burned air and fuel to be
  exhausted past the O2S. The O2S sees the
  oxygen (not the unburned gasoline) and the O2S
  voltage is low.

83

• False O2S Readings
• False Lean (continued)
• Exhaust leak in front of the O2S. An exhaust
  leak between the engine and the oxygen sensor
  causes outside oxygen to be drawn into the
  exhaust and past the O2S. This oxygen is read
  by the O2S and produces a lower than normal
  voltage. The computer interrupts the lower than
  normal voltage signal from the O2S as meaning
  that the air-fuel mixture is lean. The computer
  will cause the fuel system to deliver a richer
  air-fuel mixture.

84

• False O2S Readings (continued)
• False Lean (continued)
• A spark plug misfire represents a false lean
  signal to the oxygen sensor. The computer does
  not know that the extra oxygen going past the
  oxygen sensor is not due to a lean air-fuel
  mixture. The computer commands a richer mixture,
  which could cause the spark plugs to foul,
  increasing the rate of misfirings.

85

• False O2S Readings (continued)
• False Rich
• False rich indication (high O2S readings) can be
  attributed to the following
• Contaminated O2S due to additives in the engine
  coolant or due to silicon poisoning
• A stuck open EGR valve (especially at idle)

86

• False O2S Readings (continued)
• False Rich (continued)
• A spark plug wire too close to the oxygen sensor
  signal wire, which can induce a higher than
  normal voltage in the signal wire thereby
  indicating to the computer a false rich condition
• A loose oxygen sensor ground connection, which
  can cause a higher than normal voltage and a
  false rich signal

87

• False O2S Readings (continued)
• False Rich (continued)
• A break or contamination of the wiring and its
  connectors, which could prevent reference oxygen
  from reaching the oxygen sensor resulting in a
  false rich indication (All oxygen sensors require
  an oxygen supply inside the sensor itself for
  reference to be able to sense exhaust gas oxygen.)

88

• Post Catalytic Converter Oxygen Sensor Testing
• A changing air-fuel mixture is required for the
  most efficient operation of the converter.
• If the converter is working correctly, the oxygen
  content after the converter should be fairly
  constant.

89

• What types of sensors are the CTS and IAT?
• Where are these sensors found?
• How many wires are used for these sensors and
  what are they used for?
• How are these sensors tested?

90

• What type of sensor is a TPS?
• How many wires are found on a TPS and what are
  they used for?
• What is the voltage range of the TPS signal?
• How can a TPS be tested?
• What information does the TPS provide to the PCM?
• What does the PCM use this information for?

91

• What is the purpose of the MAP sensor?
• Where is it located?
• How is this sensor tested?
• How does a MAF differ from a MAP sensor?
• How is a MAF sensor tested?

92

• What are the main types of oxygen sensors
  currently used?
• How do they differ?
• How many wires are found on oxygen sensors and
  what are they used for?
• What tools should be used for testing oxygen
  sensors?
• How do oxygen levels in the exhaust affect sensor
  output?