Due to emissions regulations, the vehicle must enter Closed Loop as soon as possible. By heating the individual oxygen sensors, they reach their activation temperature sooner than those without heaters. Once an oxygen sensor reaches its activation temperature, the sensor is able to monitor the oxygen content of the exhaust more accurately. The vehicle operates in Closed Loop status once activation temperature is reached. Thus the VCM fuel trim corrections reduce the vehicle total emissions. Vehicles produce less emissions when they are in Closed Loop than Open Loop.

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 in order to produce a voltage output. This voltage should constantly fluctuate from approximately 100 mV, which is high oxygen content or a lean mixture, to 900 mV, which is low oxygen content or a rich mixture.

The vehicle control module (VCM) calculates what fuel mixture command to give to the injectors based upon voltage output of the oxygen sensor. A lean mixture, based on low HO2S voltage, equals a rich command. A rich mixture, based on high HO2S voltage, equals a lean command. The HO2S voltage can be monitored with a scan tool.

Depending upon the configuration of the vehicle exhaust system, the number of oxygen sensors may vary. The pre-catalyst, HO2S bank 1 sensor 1 and bank 2 sensor 1, are positioned between the exhaust manifold and catalytic converter. The post-catalyst, HO2S bank 1 sensor 2, bank 1 sensor 3, and bank 2 sensor 2, are positioned between the catalytic converter and the muffler.

When the VCM detects an HO2S signal circuit that is low, the VCM will set one of the following diagnostic trouble codes (DTCs):

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

The VCM has the ability to monitor HO2S switching, transition time, and incorrect voltage ratio problems. When the VCM detects sluggish HO2S activity, the VCM will set one of the following DTCs:

When the VCM detects no HO2S activity, the VCM will set one of the following DTCs:

A fault in the HO2S heater element, ignition voltage, or ground will result in an increase in time to Closed Loop fuel control. This may cause increased emissions, especially during start-up. When the VCM detects a malfunction in the HO2S heater circuits, one of the following DTCs will set:

In order to control the emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), a three-way catalytic converter (TWC) is used. The converter changes the HC and CO present in the exhaust gas into harmless water vapor and carbon dioxide through a reaction with the catalyst elements. Similarly, NOx is converted into nitrogen and oxygen.

The VCM monitors this process by comparing the activity of a pre-catalyst, HO2S bank 1 sensor 1 and bank 2 sensor 1, to that of its corresponding post-catalyst, HO2S bank 1 sensor 2, bank 1 sensor 3, and bank 2 sensor 2. A pre-catalyst, HO2S bank 1 sensor 1 and bank 2 sensor 1, produces an output signal relative to the amount of oxygen present in the exhaust gas entering the three-way catalytic converter. A post-catalyst, HO2S bank 1 sensor 2, bank 1 sensor 3, and bank 2 sensor 2, produces an output signal relative to the amount of oxygen present in the exhaust gas exiting the three-way catalytic converter. The difference of the two values indicates the oxygen storage capacity of the catalyst. This in turn indicates the ability of the catalyst to convert the exhaust gases efficiently. A properly operating three-way catalytic converter (TWC) pre-catalyst oxygen sensor produces a far more active signal than that of its corresponding post-catalyst oxygen sensor.

Fuel trim values are based upon the HO2S sensor values received by the VCM during Closed Loop. The VCM adjusts the fuel trim in order to ensure that the fuel delivery is correct for catalyst monitoring.

(1)	Vehicle Control Module (VCM)
(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 vehicle control module (VCM) calculates the throttle position. As the throttle valve angle is changed, or 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 output increases so that at wide open throttle (WOT), the output voltage should be above 4.0 volts.

The VCM calculates the fuel delivery based on the throttle valve angle, or 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 VCM detects the throttle is moving.

When the VCM detects a malfunction with the TP sensor circuits, one of 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

Knock Sensor (KS) Sensor

The knock sensor (KS) system is used in order to detect the engine detonation. The VCM will retard the spark timing based on the signals from the KS module. The knock sensors produce an AC voltage that is sent to the KS module. The amount of the AC voltage produced is proportional to the amount of knock.

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

A/C Request Signal

The A/C request circuit signals the VCM when an A/C mode is selected at the A/C control head. The VCM 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. If this signal is not available to the VCM, the A/C compressor will be inoperative.

Refer to Air Conditioning (A/C) Compressor Control Circuit Diagnosis for A/C wiring diagrams and diagnosis of the A/C electrical system.

Vehicle Speed Sensor (VSS)

The vehicle speed sensor (VSS) is a pulse counter type input that informs the VCM how fast the vehicle is being driven. The VSS system uses an inductive sensor mounted in the tail housing of the transmission and a toothed reluctor wheel on the tail shaft. 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 the vehicle speed. The VCM processes this signal and sends it to the following components:

•	The instrument panel

•	The cruise control module

Crankshaft Position (CKP) Sensor

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

The VCM also monitors the CKP sensor signal circuit for malfunctions. The VCM monitors CKP signal and the high and low resolution signals. The VCM calculates these signals in order to determine a ratio. When the VCM detects that the ratio is out of normal operating range, the VCM will set a DTC P0337 or a DTC P0338.

Camshaft Position (CMP) Sensor

The camshaft position (CMP) sensor is located within the distributor. The operation of the CMP sensor is very similar to the crankshaft position (CKP) sensor. The CMP sensor will provide one pulse per camshaft revolution (1x signal). The loss of this signal may not affect the driveability of the vehicle, but will affect the type of control the VCM has on the fuel injection system. If this signal is lost, the VCM will pulse the injectors bank to bank instead of each individual injector at a time. The VCM also utilizes this signal in conjunction with the crankshaft position in order to determine which cylinders are misfiring.