The EVAP Vacuum Switch is used by the PCM to monitor EVAP purge valve operation and purge system integrity. The EVAP Vacuum Switch should be closed with no vacuum present (0% EVAP Canister Purge PWM). With EVAP Canister Purge PWM at 25% or greater, the EVAP Vacuum Switch should open.

An incorrect EVAP Purge system flow should set a DTC P0441. A continuous purge condition with no purge commanded by the PCM should set a DTC P1441.

Refer to EVAP Control System (G Car, C Car, and H Car VIN 1) for a complete description of the EVAP system.

The fuel control Heated Oxygen Sensor (HO2S 1) is mounted in the exhaust manifolds 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.

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.

Contamination of the heated oxygen sensor can result from the use of an inappropriate RTV sealant (not oxygen sensor safe), coolant consumption, or oil consumption. To check for contamination, remove the HO2S and visually inspect the portion of the sensor exposed to the exhaust stream. A contaminated HO2S is indicated by abnormal discoloration of the portion of the sensor exposed to the exhaust stream. Silicon contamination causes a high but false HO2S signal voltage (rich exhaust indication). This condition causes the PCM to reduce the amount of fuel delivered to the engine, causing a severe driveability problem.

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 engine is cold, and rise as 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.

This is a ground circuit for the digital RPM counter inside the PCM, but the wire is connected to engine ground only through the ignition control module. Although this circuit is electrically connected to the PCM, it is not connected to ground at the PCM. The PCM compares voltage pulses on the 18X and 3X reference input circuits to any on this circuit, ignoring pulses that appear on both. If the circuit is open, or connected to ground at the PCM, it may cause poor engine performance and possibly a MIL with no DTC set.

The knock sensors detect abnormal vibration (spark knocking) in the engine. The sensors are mounted in the engine block near the cylinders. The sensors produce an AC output voltage which increases with the severity of the knock. This signal voltage is input to the PCM. The PCM then adjusts the Ignition Control (IC) timing to reduce spark knock.

The PCM contains a replaceable Knock Sensor (KS) module. The KS module contains the circuitry that allows the PCM to utilize the KS signal and diagnose the KS sensors and circuitry. If the PCM is replaced, the KS module needs to be transferred from the original PCM. If the KS module is missing or faulty causing a continuous knock condition to be indicated, the PCM will set DTC P0325.

DTCs P0325, P0326, and P0327 are designed to diagnose the KS module, the knock sensors, and related wiring, so problems encountered with the KS system should set a DTC.

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

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, 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 (gm/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 values should remain fairly stable at any given RPM. A MAF sensor malfuction or MAF signal circuit problem should set DTC P0101, DTC P0102, or DTC P0103.

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 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. Refer to Automatic Transaxle Diagnosis for a complete description, diagnosis, and on-vehicle service.

The transaxle fluid temperature sensor is a thermistor which changes value based on the temperature of the Transaxle fluid. A high transaxle fluid temperature 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 on engine coolant temperature will be substituted for the TFT sensor value, and the Transaxle will operate normally. Refer to Automatic Transaxle Diagnosis for a complete description, diagnosis, and on-vehicle service.

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 TP sensor voltage increases so that at Wide Open Throttle (WOT), the TP sensor 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 or DTC P0123. A hard failure with the TP sensor ground circuit may set DTCs P0123 and 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.

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, EGR and IAC valve operation. The combination of the four 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 X=high voltage level, 0=low voltage level. The four parameters represent trasaxle range switch Parity, A, B, and C inputs respectively. Refer to Transaxle Range Switch Valid Combinations Table.