The heated oxygen sensors (HO2S) are mounted in the exhaust system where they 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).

When the PCM detects an HO2S signal circuit that is low, the PCM will set the following DTCs:

When the PCM detects an HO2S signal circuit that is high, the PCM will set the following DTCs:

When the PCM detects no HO2S activity, the PCM will set the following DTCs:

A fault in the heated oxygen sensor heater element or its ignition feed or ground will result in an increase in time to Closed Loop fuel control. This may cause increased emissions, especially at start-up. When the PCM detects a malfunction in the HO2S heater circuits, the following DTCs will set:

The PCM also has the ability to detect the following HO2S problems:

The PCM stores a DTC that indicates degraded HO2S performance if any of the above is detected.

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 Bank 1 HO2S 2 and the Bank 2 HO2S 2 heated oxygen sensors. The front HO2S sensors produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the three-way catalytic converter. The rear HO2S sensors produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the catalysts ability to convert exhaust gases efficiently. If the catalyst is operating efficiently, the front sensors will produce a far more active signal than that produced by the rear sensors.

The catalyst monitor sensors operate the same as the fuel control sensors. Although the Bank 1 HO2S 2 and Bank 2 HO2S 2 sensors main function is catalyst monitoring, they also play a limited role in fuel control. If a 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

The Accelerator Pedal Position (APP) sensor is mounted on the accelerator pedal assembly. The sensor is actually three individual Accelerator Pedal Position sensors within one housing. Three separate signal, ground and 5 volt reference circuits are used to connect the Accelerator Pedal Position Sensor assembly and the Throttle Actuator Control (TAC) Module. Each sensor has a unique functionality. The APP sensor 1 signal increases as the accelerator pedal is depressed, from below 1 volt at 0% pedal travel (pedal at rest) to above 3.3 volts at 100% pedal travel (pedal fully depressed). The APP sensor 2 signal decreases from above 4 volts at 0% pedal travel to below 1.8 volts at 100% pedal travel. The APP sensor 3 signal also decreases from above 3.5 volts at 0% pedal travel to below 2.6 volts at 100% pedal travel. Note also that the signal circuits for APP Sensor 2 and APP Sensor 3 are pulled up to 5 volts and the APP Sensor 1 signal circuit is pulled to ground within the TAC Module.

When the PCM detects a malfunction with the APP sensor circuits, the following DTCs will set:

The Throttle Position (TP) sensor is mounted on the throttle body assembly. The sensor is actually two individual Throttle Position sensors within one housing. Two separate signal, ground and 5 volt reference circuits are used to connect the TP sensor assembly and the Throttle Actuator Control (TAC) Module. The two sensors have opposite functionality. The TP sensor 1 signal voltage increases as the throttle opens, from below 1.1 volts at 0% throttle to above 3.7 volts at 100% throttle. The TP sensor 2 signal voltage decreases from above 3.9 volts at 0% throttle to below 1.2 volts at 100% throttle. Note also that the signal circuit for TP Sensor 1 is pulled up to 5 volts and that the signal circuit for TP Sensor 2 is pulled to ground within the TAC Module. The TAC module converts these different signals to a common scale and continuously compares them to each other to verify proper system operation.

When the PCM detects a malfunction with the TP sensor circuits, the following DTCs will set:

The PRND Switch is mounted to the side of the transaxle and is part of the PNP switch. The PRND switch is used by the PCM to indicate the actual gear selected. The PRND is made up of four individual switches. The scan tool will indicate ON and OFF. The combination of ONs and OFFs will indicate the gear selected. If the combination of ONs and OFFs is invalid, a DTC will set.

The EVAP Purge Vacuum Switch is used by the PCM to monitor EVAP canister purge solenoid operation and purge system integrity. The EVAP Purge Vacuum Switch should be closed to ground with no vacuum present (0% EVAP Purge PWM). With EVAP Purge PWM at 25% or greater, the EVAP Purge 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 Evaporative Emission (EVAP) Control System for a complete description of the EVAP system.

The Knock Sensor (KS) system is used to detect engine detonation. The PCM will retard the spark timing based on the signals from the KS system. The Knock Sensor(s) produce an AC voltage that is sent to the PCM. The amount of AC voltage produced is proportional to the amount of knock.

An operating engine produces a normal amount of engine mechanical vibration (Noise). The knock sensor(s) will produce an AC voltage signal from this Noise. When an engine is operating, the PCM will learn the minimum and maximum frequency of the noise the engine produces. When the PCM determines that this frequency is less than or greater than the expected amount, a knock sensor DTC will set.

The PCM uses the Fuel Level sensor input in order to determine the amount of fuel in the fuel tanks. The PCM disables the engine misfire diagnostic when the fuel level is too low. The PCM sends the fuel level information to the BCM via the Class 2 circuit. The BCM uses the Fuel Level sensor information in order to control the fuel gauge.

The A/C request circuit signals the PCM when an A/C mode is selected at the A/C control head. The PCM uses this information enable the A/C compressor clutch and 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 will be inoperative. For vehicles equipped with CJ2 A/C, the A/C request is sent via Class 2 Serial Data communications.

Refer to A/C Clutch Circuit Diagnosis for A/C wiring diagrams and diagnosis of A/C electrical system.

This signal is used by the PCM to enable the cooling fans when head pressure reaches a predetermined amount. If a fault is present in the A/C refrigerant pressure sensor circuit, DTC P0530 sets. When the PCM sets a DTC, the A/C compressor clutch will be disabled. If the A/C refrigerant pressure sensor is stuck, Refer to the A/C Refrigerant Pressure Sensor Circuit Check.

The Vehicle Speed Sensor (VSS) is a pulse counter type input that informs the PCM how fast the vehicle is being driven. The VSS system uses an inductive sensor mounted to differential housing and a toothed reluctor attached to the ring gear. As the reluctor rotates, the teeth alternately interfere with the magnetic field of the sensor creating an induced voltage pulse.

The VSS produces an AC voltage signal that increases with vehicle speed. The PCM processes this signal and sends it to various components.

The crankshaft position sensor provides the PCM with crankshaft speed and crankshaft position. The PCM utilizes this information to determine if an engine Misfire is present. The PCM monitors the CKP sensor for momentarily drop in crankshaft speed to determine if a misfire is occurring. When the PCM detects a misfire, a DTC P0300 will set.

The PCM also monitors the CKP sensor signal circuit for malfunctions. When the PCM detects a CKP sensor that is out of normal operating range, the PCM will set a DTC P0335 or a DTC P0336.

The Camshaft Position sensor is mounted through the top of the engine block at the rear of the valley cover. The Camshaft Position sensor works in conjunction with a 1X reluctor wheel on the camshaft. The reluctor wheel is inside the engine immediately in front of the rear cam bearing. The PCM provides a 12 volt power supply to the CMP sensor as well as a ground and a signal circuit.

The Camshaft Position sensor is used to determine whether a cylinder is on a firing or exhaust stroke. As the camshaft rotates, the reluctor wheel interrupts a magnetic field produced by a magnet within the sensor. The sensors internal circuitry detects this and produces a signal which is read by the PCM. The PCM uses this 1X signal in combination with the Crankshaft Position sensor 24X signal to determine crankshaft position and stroke. This diagnostic for the Camshaft Position sensor checks for a loss of Camshaft Position sensor signal. The PCM also monitors the CMP sensor signal circuit for malfunctions. The following DTCs set when the PCM detects a CMP sensor that is out of the normal operating range.