The Heated Oxygen Sensors 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. 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) by monitoring the voltage output of the oxygen sensors.

The PCM sets the following DTCs when the PCM detects an HO2S signal circuit that is low:

The PCM sets the following DTCs when the PCM detects an HO2S signal circuit that is high:

The PCM sets the following DTCs when the PCM detects no HO2S activity:

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

The PCM also has the ability to detect HO2S response, switching, transition time, and incorrect ratio voltage problems. The PCM stores a DTC that indicates degraded HO2S performance if a HO2S response switching, transition time, or ratio problem 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 Throttle Position (TP) sensor is a potentiometer. The TP sensor is connected to the throttle shaft on the throttle body. The PCM calculates throttle position by monitoring the voltage on the signal line. The TP sensor signal changes as the throttle valve angle is changed (accelerator pedal moved). The TP sensor signal voltage is low at a closed throttle position. The TP sensor signal voltage increases as the throttle valve opens so that at Wide Open Throttle (WOT), the output voltage should be above 4.0 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. This may cause an unstable idle because the PCM detects the throttle is moving.

The following DTCs set when the PCM detects a malfunction with the TP sensor circuits:

The PNP switch indicates to the PCM when the transmission is in park, neutral, or drive. This information is used for the EGR and IAC valve operation. Refer to Electrical Diagnosis for more information on the PNP switch. The PNP switch is part of the neutral/start and backup light switch assembly. Refer to Park/Neutral Position Switch diagnosis.

Important:: Idle quality will be affected if the vehicle is driven with the PNP switch disconnected. Having the switch disconnected may also cause a VSS DTC to set.

The PCM uses the fuel level sensor input in order to determine the amount of fuel in the fuel tank. The PCM disables the engine misfire diagnostic when the fuel level is too low and the EVAP system diagnostic when the fuel level is either too high or too low. The PCM also controls the fuel gauge based on the fuel level input.

The Fuel Tank Pressure sensor is a sensor much like that of the common MAP sensor. The Fuel Tank Pressure sensor measures the difference between the outside air pressure and the air pressure (or vacuum) in the fuel tank.

The Fuel Tank Pressure sensor mounts to the fuel tank sending unit. A three wire electrical harness connects the sensor to the PCM. The PCM supplies a 5.0 volt reference voltage and a ground to the sensor. The sensor will return a voltage between 0.1 and 4.9 volts on the signal circuit.

A Knock Sensor (KS) system is used in order to control spark knock. The KS system is designed to retard spark timing up to 20° in order to reduce spark knock in the engine. This allows the engine to use maximum spark advance to improve driveability and fuel economy.

The knock sensor system is used to detect engine detonation. The knock sensors produce an AC voltage which is sent to the PCM. The PCM retards the spark timing based on signals from the KS sensors. The amount of AC voltage produced by the sensors is determined by the amount of knock. The PCM then adjusts the Ignition Control (IC) to reduce the spark knock.

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 in order to enable the A/C compressor clutch and to adjust the idle speed before turning ON the A/C clutch. The PCM disables the A/C compressor clutch if the A/C request signal is not available to the PCM. 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 in order to enable the cooling fans when the A/C compressor head pressure reaches a predetermined value. DTC P0530 sets if a fault is present in the A/C refrigerant pressure sensor circuit. The PCM disables the A/C compressor clutch when the PCM sets an A/C system DTC. Refer to the A/C Refrigerant Pressure Sensor Circuit Check if the A/C refrigerant pressure sensor is stuck.

The Vehicle Speed Sensor (VSS) is a pulse counter type input that informs the PCM how fast the vehicle is traveling. The VSS system uses an inductive sensor mounted in the tail housing of the transmission and a toothed reluctor wheel on the tail shaft. The teeth of the reluctor wheel alternately interfere with the magnetic field of the sensor creating an induced voltage pulse as the reluctor rotates.

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

The crankshaft position sensor provides the PCM with crankshaft speed and crankshaft position. The PCM utilizes this information in order to determine if an engine misfire is present. The PCM monitors the CKP sensor for momentarily drop in crankshaft speed in order to determine if a misfire is occurring. A DTC P0300 sets when the PCM detects a misfire.

The PCM also monitors the CKP sensor signal circuit for malfunctions. The PCM sets a DTC P0335 or a DTC P0336 when the CKP sensor is out of the normal operating range.

The Camshaft Position sensor is mounted through the top of the engine block at the rear of the valley cover and 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 PCM uses the Camshaft Position sensor in order to determine whether a cylinder is on a firing or exhaust stroke. The reluctor wheel interrupts a magnetic field produced by a magnet within the CMP sensor as the camshaft rotates. The CMP sensor's internal circuitry detects this and produces a signal which is the PCM reads. The PCM uses this 1X signal in combination with the Crankshaft Position sensor 24X signal in order to determine the 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.