All of the sensors and input switches can be diagnosed
using a scan tool. Following is a short description of how the sensors and
switches can be diagnosed by using a scan tool. The scan tool
can also be used to compare the values for a normal running
engine with the engine you are diagnosing.
Engine Coolant Temperature (ECT) Sensor
The engine coolant temperature sensor is a thermistor (a resistor which
changes value based on temperature) mounted in the engine coolant stream.
Low coolant temperature produces a high resistance (100,000ohms
at -40°C/-40°F) while high temperature causes low resistance
(70ohms at 130°C/266°F).
The PCM supplies a 5 volt signal to the engine coolant temperature
sensor through a resistor in the PCM and measures the voltage. The voltage
will be high when the engine is cold, and low when the engine
is hot. By measuring the voltage, the PCM calculates the engine
coolant temperature. Engine coolant temperature affects most
systems the PCM controls.
The scan tool displays engine coolant temperature in degrees. After
engine startup, the temperature should rise steadily to about 90°C (194°F)
then stabilize when thermostat opens. If the engine has not
been run for several hours (overnight), the engine coolant
temperature and intake air temperature displays should be
close to each other. A hard fault in the engine coolant sensor
circuit should set DTC P0117 or DTC P0118, an intermittent
fault should set a DTC P1114 or P1115. The DTC Diagnostic
Aids also contains a chart to check for sensor resistance
values relative to temperature.
The ECT sensor (3) also contains another circuit which is used to operate
the engine coolant temperature gauge located in the instrument panel.
Mass Air Flow (MAF) Sensor
The Mass Air Flow (MAF) sensor measures the amount of air which passes
through it. The PCM uses this information to determine the operating
condition of the engine, to control fuel delivery.
A large quantity of air indicates acceleration, while a small
quantity indicates deceleration or idle.
The scan tool reads the MAF value and displays it in grams per second
(gm/s). At idle, it should read between 4gm/s to 6gm/s on a fully warmed
up engine. Values should change rather quickly on acceleration,
but values should remain fairly stable at any given RPM.
A failure in the MAF sensor (1) or circuit should set DTC P0101,
DTC P0102, or DTC P0103.
Intake Air Temperature (IAT) Sensor
The Intake Air Temperature (IAT) sensor is a thermistor which changes
value based on the temperature of air entering the engine. Low temperature
produces a high resistance (100,000 ohms at -40°C/-40°F),
while high temperature causes low resistance (70 ohms at 130°C/266°F).
The PCM supplies a 5 volt signal to the sensor through a
resistor in the PCM and measures the voltage. The voltage will be high when
the incoming air is cold, and low when the air is hot. By measuring the
voltage, the PCM calculates the incoming air temperature. The IAT sensor
signal is used to adjust spark timing according to incoming air density.
The scan tool displays temperature of the air entering the engine,
which should read close to ambient air temperature when the engine is cold,
and rise as the underhood temperature increases.
If the engine has not been run for several hours (overnight) the IAT
sensor temperature and engine coolant temperature should read close to each
other. A failure in the IAT sensor circuit should set DTC P0112
or DTC P0113.
Manifold Absolute Pressure (MAP) Sensor
The Manifold Absolute Pressure (MAP) sensor responds to changes in
intake manifold pressure (vacuum). The MAP sensor signal voltage to the PCM
varies from below 2 volts at idle (high vacuum) to above 4 volts
with the key on, engine not running or at wide open throttle (low
vacuum).
The MAP sensor is used to determine manifold pressure changes while
the linear EGR flow test diagnostic is being run (refer to DTC P0401),
to determine engine vacuum level for other diagnostics and to determine
barometric pressure (BARO).
If the PCM detects a voltage that is lower than the possible range
of the MAP sensor, DTC P0107 will be set. A signal voltage higher than
the possible range of the sensor will set DTC P0108.
An intermittent low or high voltage will set DTC P1107
or P1106 respectively. The PCM can also detect a shifted
MAP sensor. The PCM compares the MAP sensor signal to a calculated
MAP based on throttle position and various engine load factors
If the PCM detects a MAP signal that varies excessively
above or below the calculated value, DTC P0106 will
set.
Fuel Control Heated Oxygen Sensor (HO2S 1)
The fuel control Heated
Oxygen Sensor (HO2S 1) is mounted in the exhaust manifold where it
can monitor the oxygen content of the exhaust gas stream.
The oxygen present in the exhaust gas reacts with the sensor
to produce a voltage output. This voltage should constantly
fluctuate from approximately 100mV (high oxygen content
lean mixture) to 900mV (low oxygen content rich mixture).
The heated oxygen sensor voltage can be monitored with a
scan tool. By monitoring the voltage output of the oxygen sensor,
the PCM calculates what fuel mixture command to give to
the injectors (lean mixture low HO2S voltage = rich command,
rich mixture high HO2S voltage = lean command).
The HO2S 1 circuit, if open, should set a DTC P0134 and
the scan tool will display a constant voltage between 400 - 500mV. A
constant voltage below 300mV in the sensor circuit (circuit
grounded) should set DTC P0131, while a constant voltage
above 800mV in the circuit should set DTC P0132. A fault
in the HO2S 1 heater circuit should cause DTC P0135
to set. The PCM can also detect HO2S response problems. If
the response time of an HO2S is determined to be too slow,
the PCM will store a DTC that indicates degraded HO2S performance.
Catalyst Monitor Heated Oxygen Sensor (HO2S 2)
To control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and
Oxides of Nitrogen (NOx), a three way catalytic converter
is used. The catalyst within the converter promotes a chemical
reaction which oxidizes the HC and CO present in the exhaust
gas, converting them into harmless water vapor and carbon dioxide.
The catalyst also reduces NOx, converting it to nitrogen.
The PCM has the ability to monitor this process using the
HO2S 1 and the HO2S 2 heated oxygen sensors.
The HO2S 1 sensor produces an output signal which
indicates the amount of oxygen present in the exhaust gas entering
the three way catalytic converter. The HO2S 2 sensor
produces an output signal which indicates the oxygen storage
capacity of the catalyst, this in turn indicates the catalyst's
ability to convert exhaust gases efficiently. If the catalyst
is operating efficiently, the HO2S 1 signal will be
far more active than that produced by the HO2S 2 sensor.
The catalyst monitor sensors operate the same as the fuel control sensors.
Although the HO2S 2 sensors' main function is catalyst monitoring,
it also plays a limited role in fuel control. If the sensor
output indicates a voltage either above or below the 450
millivolt bias voltage for an extended period of time,
the PCM will make a slight adjustment to fuel trim to ensure
that fuel delivery is correct for catalyst monitoring.
A problem with the HO2S 2 signal circuit should set DTC P0137,
P0138 or P0140, depending on the specific condition. A fault in the heated
oxygen sensor heater element or its ignition feed or ground
will result in slower oxygen sensor response. This may
cause erroneous Catalyst monitor diagnostic results. A
fault in the HO2S 2 heater circuit should cause DTC P0141
to set.
Throttle Position (TP) Sensor
The Throttle Position
(TP) sensor is a potentiometer connected to the throttle shaft on the throttle
body. By monitoring the voltage on the signal line, the
PCM calculates throttle position. As the throttle valve
angle is changed (accelerator pedal moved), the TP sensor
signal also changes. At a closed throttle position, the output
of the TP sensor is low. As the throttle valve opens, the
output increases so that at Wide Open Throttle (WOT), the
output voltage should be above 4 volts.
The PCM calculates fuel delivery based on throttle valve angle (driver
demand). A broken or loose TP sensor may cause intermittent bursts of fuel
from an injector and unstable idle because the PCM thinks
the throttle is moving. A hard failure in the TP sensor 5
volts reference or signal circuits should set either a DTC P0122
P0123. A hard failure with the TP sensor ground circuit may
set DTCs P0107, P0112, P0123 or P0117. Once a DTC is set, the
PCM will use an artificial default value based on engine
RPM and mass air flow for throttle position and some vehicle
performance will return. A high idle may result when either
DTC P0122 or DTC P0123 is set.
The PCM can detect intermittent TP sensor faults. DTC P1121
or DTC P1122 will set if an intermittent high or low circuit failure
is being detected. The PCM can also detect a shifted TP sensor.
The PCM monitors throttle position and compares the actual
TP sensor reading to a predicted TP value calculated from
engine speed. If the PCM detects an out of range condition,
DTC P0121 will be set.
EGR Pintle Position Sensor
