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 (100,000 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 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 start-up, 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.
Refer to
Temperature Versus Resistance
.
The ECT sensor 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)
(1) 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-7gm/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
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 (5) 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 (5) is used to determine manifold pressure changes while
the linear EGR flow test diagnostic is being run ), refer to
DTC P0401 Exhaust Gas Recirculation (EGR) Flow Insufficient
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.
The PCM updates the MAP sensor reading at each 3X reference pulse.
If the 3X reference pulse is lost the PCM will only update the MAP sensor
once per ignition cycle and will retain that value until the next ignition
cycle. Depending on the retained MAP sensor value, the PCM will set the
appropriate high voltage DTC or low voltage DTC.
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 100 mV
(high oxygen content--lean mixture) to 900 mV (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 400-500 mV.
A constant voltage below 300 mV in the sensor circuit (circuit grounded)
should set DTC P0131, while a constant voltage above 800 mV 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, P1350. 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