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

(1)	Powertrain Control Module (PCM)
(2)	Throttle Position (TP) Sensor
(3)	Throttle Valve

The Throttle Position (TP) sensor is a potentiometer. The TP sensor is 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 4.0 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.

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

•	DTC P0121 circuit performance malfunction.

•	DTC P0122 circuit low.

•	DTC P0123 circuit high.

•	DTC P1121 intermittent circuit high.

•	DTC P1122 intermittent circuit low.

EVAP Purge Vacuum Switch

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.

Knock Sensors (KS)

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 module. The Knock Sensor(s) produce an AC voltage that is sent to the KS module. 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.

A/C Request Signal

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.

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

A/C Refrigerant Pressure Sensor

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 or P0531 will set. When the PCM sets a DTC, the A/C compressor clutch will be disabled.

Power Steering Pressure (PSP) Switch