The fuel control (2) heated oxygen sensor (HO2S 1) is mounted in the exhaust pipe below the exhaust manifold. The main function of the fuel control heated oxygen sensor is to provide the powertrain control module (PCM) with exhaust stream oxygen content information. The exhaust stream oxygen content information enables the PCM to provide the proper fueling and achieve vehicle emissions that are within the mandated levels. The HO2S 1 consists of the following components:

The HO2S 1 has a zirconia element with a thin platinum surface coating. The zirconia element generates an electromotive force when a there is a difference in the concentration of oxygen between the element's faces. This electromotive force is amplified by the catalytic reaction of the platinum when the zirconia element temperature rises. The inside of the zirconia element is exposed to the atmosphere (reference air) and the outside of the zirconia element is exposed to the exhaust gases. The difference in concentration between the inside and the outside of the zirconia element varies with the concentration of oxygen in the exhaust gases. A large difference in the concentration of oxygen results in about 1 volt of electromotive force. A small difference in the concentration of oxygen results in a about 0.01 volt of electromotive force.

In order for the HO2S 1 to function properly, the sensor must have a supply of clean reference air. Clean reference air is obtained through the oxygen sensor pigtail wiring. Any attempt to repair the wires, the connectors, or the terminals of the HO2S 1 pigtail wiring could result in the obstruction of the reference air. Replace the oxygen sensor if the pigtail wiring, the connector, or the terminals are damaged.

The oxygen sensor heater greatly decreases the amount of time required for the HO2S 1 to become active and begin the closed loop fuel control.

The HO2S 1 voltage should constantly fluctuate from approximately 100 mV to 900 mV . The PCM calculates what fuel mixture commands to send to the fuel injectors by monitoring the voltage output of the oxygen sensor. The oxygen sensor voltage can be monitored with a scan tool.

The oxygen sensor's ability to provide accurate and useful voltage signals can be affected by the presence of certain contaminants. The contaminants can be introduced through the fuel system or the contaminants can be airborne. Some of the contaminants that may be encountered are phosphorus, lead, silica, and sulfur. One of the more common contaminants is silica in the form of silicone. Silicone contamination may be indicated by a white powdery deposit on the portion of the HO2S that is exposed to the exhaust stream. Silicone contamination can be caused by the use of gasoline with silicone in it or by the use of RTV sealants which emit silicone into the crankcase or induction system. Oxygen sensors exposed to high concentrations of engine coolant or engine oil in the exhaust stream can also be adversely affected.

The fuel control HO2S 1 is diagnosed for the following conditions:

The post catalyst (1) heated oxygen sensor 2 (HO2S 2) is located in the exhaust pipe after the catalytic converter. The powertrain control module (PCM) uses the HO2S 2 in order to monitor the oxygen storage capability of the catalytic converter.

A 3-way catalytic converter (TWC) is used in order to control the emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx). The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas. The catalyst converts the HC and CO into harmless water vapor and carbon dioxide. The catalyst also reduces NOx by converting the NOx to nitrogen. The HO2S 2 reacts to the oxygen content in the exhaust stream after the exhaust passes through the catalytic converter. The voltage signal created by the HO2S 2 sensor ranges from approximately 0.1 volt to 0.9 volt. The oxygen sensor heater is required for the catalyst monitor HO2S 2 in order to become active and begin accurate catalyst monitoring. An HO2S 2 signal that appears lazy or inactive is normal. The PCM compares readings from both the HO2S 1 and the HO2S 2 in order to determine the efficiency of the catalyst in the TWC converter.

In order for the HO2S 2 to function properly, the sensor must have a supply of clean reference air. Clean reference air is obtained through the oxygen sensor pigtail wiring. Any attempt to repair the wires, the connectors, or the terminals of the HO2S 2 pigtail wiring could result in the obstruction of the reference air. Replace the oxygen sensor if the pigtail wiring, the connector or the terminals are damaged.

The oxygen sensor heater greatly decreases the amount of time required for the HO2S 1 to become active and begin the closed loop fuel control.

The catalyst monitor HO2S 2 is diagnosed for the following functions:

The engine coolant temperature (ECT) sensor is a thermistor -- a resistor whose resistance changes as a function of temperature. The ECT sensor is mounted in the engine coolant stream. The ECT sensor has 2 internal sensors, one portion (a single wire) controls cooling fan operation and is NOT controlled or monitored by the PCM. The other portion (2 wires) is PCM monitored.

The PCM monitored portion of the ECT sensor is a 2-wire sensor with a reference voltage and a ground provided by the PCM . Low coolant temperature produces a high resistance and high temperatures result in a low resistance.

The PCM supplies a 5 volt signal to the coolant sensor through a resistor in the PCM and measures the signal voltage. The voltage will be high when the engine is cold and low when the engine is hot.

The ECT sensor provides engine coolant temperature information to the PCM for fuel enrichment, the ignition timing, the air management, the idle speed control, and Closed Loop fuel control.

The intake air temperature (IAT) sensor (2) is an integral part of the mass air flow (MAF) sensor and is mounted in the air cleaner assembly. The IAT sensor measures the temperature of the air entering the intake manifold. The IAT sensor provides temperature information to the circuitry of the MAF sensor and the PCM.

The IAT sensor is a thermistor -- a resistor whose resistance changes as a function of temperature. When the temperature is low, the resistance is high. The resistance decreases as the temperature increases. The IAT sensor is a 2-wire circuit with a reference or signal voltage and a ground coming from the PCM.

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 the throttle position. As the throttle valve angle changes when 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, the output voltage should be more than 3.3 volts.

By monitoring the output voltage from the TP sensor, the PCM can modify the fuel delivery based on the throttle angle. For example, power enrichment occurs when the throttle angle approaches wide-open throttle. The PCM looks primarily for changes in the TP sensor output to control fuel delivery. Acceleration enrichment occurs when the throttle angle increases, similar to an accelerator pump on a carbureted vehicle.

The mass air flow (MAF) sensor measures the changes in the intake air volume that result from the changes in throttle opening and the air density. The airflow measurements are used by the PCM in order to determine the engine fueling requirements.

The MAF sensor is a hot-wire design. A platinum hot wire and a thermister are located in the intake air bypass passage of the MAF sensor housing. The temperature of the platinum hot wire is affected by exposure to air flow and by exposure to air temperature. The platinum hot wire is maintained at a set temperature by controlling the current flow through the wire. The MAF sensor converts the changes in current flow to a voltage signal. The voltage signal from the MAF sensor enables the PCM to detect changes in the air density and changes in air volume.

The MAF sensor also contains the IAT sensor. The IAT sensor cannot be serviced separately from the MAF sensor.

The VSS is an electronic relay that is mounted on the transaxle. As the transaxle turns the VSS, the VSS provides the speedometer with a vehicle speed input through voltage pulses. This input is used to drive the speedometer. The speedometer then converts this vehicle speed input into a more precise waveform and provides the PCM and the cruise control module, if equipped, with the vehicle speed input. The PCM converts this input (ground pulses) into a vehicle speed.

The crankshaft position (CKP) sensor is located in the front cover of the cylinder block near the crankshaft pulley. The CKP sensor produces an AC signal that increases in both frequency and amplitude as the engine speed increases. The CKP sensor signal is sent to the PCM in order to indicate the RPM and the crankshaft position. The PCM uses the CKP sensor signal along with the camshaft position (CMP) sensor signal for the following purposes:

The CKP sensor signal rotor has 34 teeth and is mounted on the crankshaft behind the timing chain cover and the crankshaft pulley. When the crankshaft rotates, the CKP sensor signal rotor teeth pass by the CKP sensor causing a fluctuation in the sensor's magnetic field. The fluctuation in the magnetic field induces a voltage in the CKP sensor circuitry. The sensor sends this signal to the PCM at rate of 34 signals per crankshaft revolution.

The camshaft position (CMP) sensor is located in the cylinder head near the number 4 fuel injector. The CMP sensor is a signal generator that is composed of a magnet and a coil with an iron core. The PCM relies on the AC signal provided by the CMP sensor in order perform the following:

The CMP sensor signal rotor (2) is part of the intake camshaft (1) and has 3 teeth located on the outer circumference. When the CMP sensor signal rotor (2) rotates past the CMP sensor (3), electrical signals are generated. The AC signals that are generated by the CMP sensor are sent to the PCM.

The fuel tank pressure sensor is located on top of the EVAP canister near the fuel tank. The fuel tank pressure sensor measures the fuel vapor pressure in the fuel tank and compares the vapor pressure with the barometric pressure. The fuel tank pressure sensor is similar to the manifold absolute pressure sensor. The PCM supplies 5 volts and a ground to the sensor. The fuel tank pressure sensor sends a voltage signal from 0.1 to 4.9 volts back to the PCM. The PCM uses the fuel tank pressure sensor signal as one of the inputs in order to detect EVAP control system malfunctions.

The knock sensor (KS) is located below the intake manifold, between cylinder 2 and cylinder 3, on the engine block. The KS (1) detects engine detonation and sends a signal to the PCM. The PCM uses the input from the KS to adjust the ignition timing in order to control detonation. The operation of the KS is detailed in the description of the knock sensor system.

The A/C compressor control module provides an A/C On signal to the PCM when the A/C compressor clutch is in operation. The PCM uses the A/C On signal in order to adjust the fuel mixture and modify the engine idle speed. The PCM will signal the idle air control valve to open the idle air passage. Opening the idle air passage slightly will increase the engine speed in order to prevent a rough idle or a stalling condition.

The PCM receives signals from several electrical components when either of the following electrical loads are present:

The PCM will increase the engine idle speed when receiving an electrical load input. The PCM will signal the idle air control (IAC) valve to open the idle air passage. Opening the idle air passage slightly will maintain an engine speed that provides a desirable idle. The PCM will return the engine to the original idle when the electrical load signal is removed.

The stoplamp switch is closed when the brake pedal is depressed. When closed, the stoplamp switch signals the PCM that the vehicle is braking. The PCM reduces the fuel cutoff speed slightly when the stoplamp switch indicates that the vehicle is braking.

The crank signal is sent from the starter motor circuit. When the starter motor circuit is energized as the ignition switch is in the START position, a crank signal is supplied to the PCM. The PCM increases the fuel injector pulse when receiving a voltage on the crank signal circuit. The slight increase in fuel provides quicker and smoother engine start-up. The crank signal is also used as an input for running certain engine control system diagnostics, such as the CKP sensor DTC P0335. The crank signal input can be monitored on a scan tool as the Starter Switch parameter.

The transaxle range switch is located on the automatic transaxle. The PCM detects the transaxle range by monitoring the ON and OFF signals from the transaxle range switch. The PCM uses the transaxle range switch signal as one of the inputs in order to control the fuel injectors, the IAC valve, and the automatic transaxle performance.

The power steering pressure (PSP) switch (1) is located in the high pressure line, near the power steering pump (2). The PSP switch signals the PCM when the vehicle is experiencing power steering assist. Turning the steering wheel causes increased power steering fluid pressure that closes the PSP switch. The PCM uses the PSP switch signal to adjust the IAC valve. In response to the PSP switch input the IAC valve increases the idle speed before the power steering load can cause an engine idle quality concern.