The ignition control module (ICM) sends the 3X reference signal to the powertrain control module (PCM). The PCM uses this signal to calculate the engine RPM and crankshaft position (CKP) at engine speeds above 1200 RPM. The PCM also uses the pulses on this circuit to initiate injector pulses. If the PCM receives no pulses on this circuit, DTC P1374 is set and the PCM uses the 18X reference signal circuit for fuel and ignition control. The engine continues to start and run using the 18X reference signal only. Refer to Electronic Ignition (EI) System Description for further information.

The ICM sends the 18X reference signal to the PCM. The PCM uses this signal to calculate engine RPM and CKP at engine speeds below 1200 RPM. The PCM also uses the pulses on this circuit to initiate injector pulses. If the PCM receives no pulses on this circuit, DTC P0336 is set and the PCM uses the 3X reference signal circuit at all times for fuel and ignition control. The engine continues to start and run using the 3X reference signal only. Refer to Electronic Ignition (EI) System Description for further information.

The A/C pressure sensor signal indicates high side refrigerant pressure to the PCM. The PCM uses this information to adjust the idle air control (IAC) valve in order to compensate for the higher engine loads present with high A/C refrigerant pressures and to control the cooling fans. The PCM also shares the A/C pressure sensor information with the HVAC controller. The HVAC controller uses the information to decide whether or not to send the A/C request signal to the PCM. If a fault occurs in the A/C pressure sensor circuit, the HVAC controller removes the request for A/C and the PCM disables the A/C relay.

The HVAC controller sends the A/C request signal to the PCM. The A/C request signal indicates to the PCM that conditions exist to allow A/C operation. The PCM uses this information to adjust the idle speed before turning on the A/C clutch. If this signal is not available to the PCM, the A/C compressor is inoperative.

The camshaft position (CMP) sensor generates the signal sent to the PCM via the ICM. The ICM filters and buffers the CMP signal. The ICM then relays the signal to the PCM. The PCM uses the CMP signal as a sync pulse in order to trigger the injectors in the proper sequence. This allows the PCM to calculate true sequential fuel injection (SFI) mode of operation. If the PCM detects an incorrect CAM signal while the engine is running, DTC P0341 is set.

If the PCM detects a loss of the CAM signal while the engine is running, the fuel injection system shifts to a calculated SFI mode. The PCM determines the fuel injector sequence based on the last fuel injection pulse received. In the calculated SFI mode, the engine continues to start and run. However, with the fault present, only a 1 in 6 chance of the correct injector sequence exists.

Refer to DTC P0341 Camshaft Position (CMP) Sensor Performance for further information.

The PCM uses the Class II serial data line to communicate with various other components and systems within the vehicle. The PCM ensures that communication remains established by monitoring state of health messages from the other devices using the circuit. If the PCM detects a loss of the state of health serial data message from the electronic brake control module (EBCM), the PCM stores DTC P1602. The EBCM disables traction control, set an ABS/TCS DTC, and illuminates the TRACTION OFF lamp.

The PCM also receives rough road information from the EBCM on the Class II serial data circuit. The PCM uses the rough road information in order to enhance the misfire diagnostic by detecting crankshaft speed variations caused by driving on rough road surfaces. This allows the PCM to reject false misfire information. The EBCM calculates rough road information by monitoring the ABS wheel speed sensors. If a malfunction occurs which does not allow the EBCM to transmit correct rough road information to the PCM while a misfire DTC is requesting the malfunction indicator lamp (MIL), DTC P1380 is set. If a loss of communications occurs which causes the PCM to not receive rough road information while a misfire DTC is requesting the MIL, DTC P1381 is set.

The engine coolant temperature (ECT) sensor is a thermistor (a resistor which changes value based on temperature) mounted in the engine coolant stream. A 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 ECT sensor through a resistor in the PCM and measures the voltage. The voltage is high when the engine is cold, and low when the engine is hot. By measuring the voltage, the PCM calculates the ECT. The scan tool displays the ECT in degrees. After engine startup, the temperature should rise steadily to about 90°C (194°F) and then stabilize when the thermostat opens. If the engine has not been run for several hours (overnight), the ECT and intake air temperature (IAT) displays should be close to each other.

The ECT affects most systems the PCM controls. A hard fault in the ECT sensor circuit should set DTC P0117 or DTC P0118; an intermittent fault should set a DTC P1114 or P1115. For sensor resistance values relative to temperature refer to Temperature vs Resistance .

The PCM monitors the EGR valve pintle position input in order to ensure that the valve responds properly to commands from the PCM and to detect a fault if the pintle position sensor circuit is open or shorted. If the PCM detects an excessively low EGR feedback signal voltage (pintle position feedback open or shorted), DTC P0405 is set.

The linear EGR valve is controlled by using an ignition positive driver and a ground circuit within the PCM. The driver has the ability to detect an electrical malfunction in the ignition positive or ground circuit. If an electrical malfunction occurs, DTC P0403 is set.

When the ignition switch is turned on, the PCM learns the EGR closed valve pintle position. When the PCM commands the EGR valve closed, the learned EGR closed valve pintle position is compared to the actual EGR position. If the actual EGR position indicates that the EGR valve is still open, DTC P1404 is set.

When the PCM commands the EGR valve open, the actual EGR position is compared with the desired EGR position. If the actual EGR position is 15% less than the desired EGR position when the PCM is commanding the EGR valve opened, DTC P1404 is set.

The engine oil level switch is a simple float switch that is grounded when the engine oil level is OK. When the ignition is first turned on, the PCM commands the Low Oil Level lamp on for approximately 3 seconds in order to test the bulb. The PCM also checks the engine oil level switch circuit at startup. If the engine has bee running, the PCM performs a test routine based on the ECT in order to ensure that the engine oil has drained back into the sump before checking the state of the engine oil level switch. If the ECT is between 15°C (59°F) and 130°C (266°F), the PCM compares the ECT at the last key off to the ECT at the current key on. If the difference between the recorded temperature values is at least 12°C (54°F), the PCM tests the engine oil level.

The PCM applies battery voltage through a pull up resistor to the engine oil pressure switch circuit. The PCM monitors the applied voltage on the engine oil pressure switch circuit. If the ignition switch is turned on with the engine not running or the engine oil pressure is below 35 kPa (5 psi), the engine oil pressure switch grounds the PCM voltage input. The PCM sends the engine oil pressure information via Class II serial data to the instrument panel cluster (IPC). The IPC controls the engine oil pressure indicator.

The PCM supplies 5 volts to the fuel level sensor circuit. The fuel level sensor varies the voltage relative to the amount of fuel in the tank. The PCM monitors the voltage in the fuel level circuit to calculate the amount of fuel in the tank. The fuel level input to the PCM is used to determine if the fuel level in the tank is correct to run the EVAP diagnostic tests. To ensure sufficient volume in the tank to begin the various diagnostic tests, the fuel level must be between 15% and 85%. Refer to Evaporative Emission Control System Operation Description for a complete description of the EVAP system. The PCM also sends the fuel level information via class 2 serial data to the instrument panel cluster (IPC). The IPC uses the fuel level information to control the IPC fuel gauge.

The fuel tank pressure sensor is used to detect vacuum decay and excess vacuum during the enhanced EVAP diagnostic routine. Refer to Evaporative Emission Control System Operation Description for a complete description of the EVAP system.

The fuel control heated oxygen sensor (HO2S 1) is mounted in the exhaust manifolds where the sensor 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 displays 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 stores a DTC that indicates degraded HO2S performance.

To control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), a 3-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 converts NOx 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 3-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 is 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, the HO2S 2 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 makes a slight adjustment to fuel trim in order to ensure that fuel delivery is correct for catalyst monitoring.

A problem with the HO2S 2 signal circuit should set DTC P0137, DTC P0138, or DTC P0140, depending upon the specific condition. A fault in the HO2S heater element, ignition feed, or ground results 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.

(1)	Throttle Body
(2)	Air Intake Duct
(3)	Intake Air Temperature (IAT) Sensor
(4)	Air Cleaner/PCM Housing Assembly

The intake air temperature (IAT) sensor (3) is a thermistor which changes value based on the temperature of air entering the engine. A low air temperature produces a high resistance (100,000 ohms at -40°C/-40°F), while a high air 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 is 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 the spark timing according to the incoming air density. The scan tool displays the temperature of the air entering the engine, which should read close to the ambient air temperature when engine is cold, and rise as underhood temperature increases. If the engine has not been run for several hours (overnight) the IAT and the ECT should read close to each other. A failure in the IAT sensor circuit should set DTC P0112 or DTC P0113.

Reference Low

The reference low circuit establishes a common ground between the ignition control module (ICM) and the PCM. The circuit minimizes electrical ground differences between the PCM and the ICM. The PCM uses the reference low circuit to clearly recognize the 3X and 18X reference signals. A malfunction in the reference low circuit may result in a reduced driveability condition and possibly a malfunction indicator lamp (MIL) activation with no DTC set.

Knock Sensors

The knock sensors (KS) detect abnormal vibration (spark knocking) in the engine. The sensors are mounted in the engine block near the cylinders. The KS produce an AC voltage signal under all engine operating conditions. The PCM adjusts the Ignition Control (IC) spark timing based on the amplitude and frequency of the KS signal being received.

The PCM contains integrated KS diagnostic circuitry. The PCM uses the circuitry to diagnose the KS sensors and related wiring. The PCM calculates an average voltage of each knock sensor's signal and performs instantaneous signal voltage readings. The PCM uses the instantaneous signal voltage readings to determine the state of the KS circuitry. If the KS system is operating normally, the PCM should monitor instantaneous KS signal voltage readings varying outside a voltage range above and below the calculated average voltage. If the PCM malfunctions in a manner which does not allow proper diagnosis of the KS circuits, DTC P0325 is set. DTCs P0327 and P0332 are designed to diagnose the knock sensors and related wiring. Problems encountered with the KS system should set a DTC.

Refer to Knock Sensor (KS) System Description for a complete description of the KS system.

Mass Air Flow Sensor

The mass air flow (MAF) sensor measures the amount of air which passes through the throttle body. The PCM uses this information to determine the operating condition of the engine, in order to control fuel delivery. A large quantity of air indicates acceleration, while a small quantity indicates deceleration or idle.

The scan tool displays the MAF value in grams per second (g/s). At idle, MAF should read between 4 gm/s - 7 gm/s on a fully warmed up engine. Values should change rather quickly on acceleration, but should remain fairly stable at any given RPM. A MAF sensor malfunction or MAF signal circuit problem should set DTC P0101, DTC P0102, or DTC P0103.

Manifold Absolute Pressure Sensor

The manifold absolute pressure (MAP) sensor responds to changes in intake manifold pressure. The PCM supplies a 5 volt reference and a ground for the MAP sensor. The MAP sensor provides a signal to the PCM relative to pressure changes in the manifold. The MAP sensor signal voltage to the PCM varies from below 2 volts at idle (low manifold absolute pressure - high vacuum) to above 4 volts with the key on and the engine not running or at wide-open throttle (high manifold absolute pressure - 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 the barometric pressure (BARO).

If the PCM detects a voltage that is lower than the possible range of the MAP sensor, DTC P0107 is set. A signal voltage higher than the possible range of the sensor sets DTC P0108. An intermittent low or high voltage sets DTC P1107 or P1106, respectively.

TCC Brake Switch

The TCC brake switch signal indicates when the brake pedal is applied. The TCC brake switch information is used by the PCM mainly to control the transaxle torque converter clutch.

Transaxle Temperature Sensor

The transaxle fluid temperature (TFT) sensor is a thermistor which changes value based on the temperature of the transaxle fluid. A high TFT may cause the vehicle to operate in Hot Mode. While in Hot Mode, shift points may be altered, 4th gear disabled, and TCC forced on in 2nd gear.

A failure in the TFT sensor or associated wiring should cause DTC P0712 or P0713 to set. In this case, a value based upon the ECT is substituted for the TFT sensor value, and the transaxle operates normally. Refer to Functional Test in Automatic Transaxle.

Throttle Position 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 the throttle position. When the throttle valve angle is changed as the accelerator pedal is 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 TP sensor voltage increases so that, at wide open throttle (WOT), the TP sensor voltage should be above 4 volts. The PCM calculates the fuel delivery based upon the 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 volt reference or signal circuits should set either a DTC P0122 or DTC P0123. A hard failure in the TP sensor ground circuit may set DTCs P0123 and P0117. Once a DTC is set, the PCM uses an artificial default value based upon engine RPM and mass air flow for the throttle position and some vehicle performance returns. 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 is set if an intermittent high or low circuit failure is detected. The PCM can also detect a shifted TP sensor. The PCM monitors the throttle position and compares the actual TP sensor reading to a predicted TP value calculated from the engine speed. If the PCM detects an out of range condition, DTC P0121 is set.

Transaxle Range Switch

The transaxle range switch is part of the transaxle park/neutral position (PNP) switch mounted on the transaxle manual shaft. The 4 inputs from the transaxle range switch indicate to the PCM which position is selected by the transaxle selector lever. This information is used for transmission shift control, ignition timing, EVAP canister purge, and EGR and IAC valve operation. The combination of the 4 transaxle range input states determine the PCM commanded shift pattern. The input voltage level at the PCM is high (B+) when the transaxle range switch is open and low when the switch is closed to ground. The state of each input is represented on the scan tool as High voltage level, Low voltage level. The 4 parameters represent transaxle range switch parity, A, B, and C inputs respectively. Valid transaxle range input combinations are shown in the Transaxle Range Switch Valid Input Combinations table.