The fuel control heated oxygen sensor 1 (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. This information enables the PCM to provide the proper fueling and achieve vehicle emissions that are within the mandated levels. The HO2S 1 consists of a zirconia element (2), a heater (3), a cover and housing assembly, and an electrical harness.

The zirconia element has a thin platinum surface coating. The zirconia element generates an electromotive force when a there is a difference in the concentration of oxygen between its 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, it must have clean reference air provided to it. This 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 (high oxygen content -- lean mixture) to 900 mV (low oxygen content -- rich mixture). 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 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 from 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 heated oxygen sensor (HO2S 1) is diagnosed for the following conditions:

A three-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 powertrain control module (PCM) uses the HO2S 2 in order to monitor the oxygen storage capability of the catalytic converter. The HO2S 2 reacts to the oxygen content in the exhaust stream after it passes through the catalytic converter. The voltage signal created by the HO2S 2 sensor ranges from approximately 0.1 volt (high oxygen -- lean mixture) to 0.9 volt (low oxygen -- rich mixture). 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, it must have clean reference air provided to it. This 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 catalyst monitor heated oxygen sensor (HO2S 2) is diagnosed for the following functions:

The engine coolant temperature (ECT) sensor (1) is located in the engine block below the engine coolant temperature sending unit (2). The ECT sensor is a thermistor (a variable resistor that changes value when the temperature changes). The ECT sensor is connected in series with a fixed resistor in the powertrain control module (PCM). The PCM applies 5 volts to the ECT sensor. The PCM monitors the voltage drop across the ECT sensor and converts the voltage reading into a temperature value. The ECT sensor voltage reading at the PCM will vary with the changes in the engine temperature.

The throttle position (TP) sensor detects the throttle valve opening. The TP sensor contains a potentiometer and a contact switch. The contact switch is known as the closed throttle position (CTP) switch. The potentiometer is connected to the throttle valve shaft on the throttle body and is responsible for sending the throttle position signal to the powertrain control module (PCM).

A 5 volt reference voltage is applied to the TP sensor from the PCM. The voltage reading at the PCM changes as the throttle plate opening increases. The PCM can calculate the throttle valve opening by monitoring the TP sensor output voltage.

The PCM uses the TP sensor signal for one of the inputs in order to control the fuel injector, the idle speed control motor and the exhaust gas recirculation solenoid vacuum valve. The PCM also converts the TP sensor voltage input into an ON/OFF signal for use by the automatic transmission.

The intake air temperature (IAT) sensor (1) is located in the air cleaner. The IAT sensor is a thermistor (a variable resistor that changes value when the temperature changes). The IAT sensor is connected in series with a fixed resistor in the powertrain control module (PCM). The PCM applies 5 volts to the IAT sensor. The PCM monitors the voltage drop across the IAT sensor and converts the voltage reading into a temperature value. The IAT sensor voltage reading at the PCM will vary with the changes in the intake air temperature.

The vehicle speed sensor (VSS) is part of the speedometer and is located in the instrument panel assembly. The VSS consists of a reed switch and a magnet. The magnet turns with the speedometer cable, causing the reed switch to turn ON and OFF. The ON and OFF frequency of the reed switch increases or decreases in proportion with the vehicle speed. The powertrain control module receives a digital signal from the VSS.

The mass air flow (MAF) sensor measures the amount of air which passes through it in a given amount of time. The powertrain control module (PCM) uses this information in order to determine the operating requirements of the engine and control fuel delivery. A large quantity of air movement indicates acceleration, while a small quantity indicates deceleration or idle.

The MAF sensor consists of a heat resistor, metering duct, straightening net, body and control circuit. This MAF sensor is of the thermal control type. The heat resistor is cooled off by the air entering the MAF sensor. The control circuit maintains the heat resistor temperature within a predetermined range. When the amount of air entering the sensor increases, the signal created by the sensor and sent to the PCM, also increases. When the air flow decreases, so does the signal current to the PCM. The MAF sensor is located in the air intake duct between the throttle body and the air cleaner.

The manifold absolute pressure (MAP) sensor (1) measures the change in the intake manifold pressure (vacuum) (2). The powertrain control module (PCM) applies 5 volts to the MAP sensor. The MAP sensor consists of a semi-conductor type pressure sensing element (3). The pressure sensing element converts a change in pressure into an electrical signal. The MAP sensor also contains electronic circuitry that amplifies and corrects the electrical signal. The PCM monitors the change in manifold pressure that results from the changes in RPM and engine load. A low MAP sensor voltage reading at the PCM indicates low manifold pressure. A high MAP sensor voltage reading at the PCM indicates high manifold pressure.

The crankshaft position (CKP) sensor consists of a magnet and a coil. The CKP sensor is mounted on the oil pan, behind the crank pulley. The CKP sensor has a specified air gap between the sensor core end and the crankshaft signal rotor. An AC voltage (pulse) is generated in the sensor when the crankshaft turns. The powertrain control module (PCM) receives a digital signal from the CKP sensor. The PCM uses the CKP sensor signal in order to determine crankshaft speed. The PCM uses crankshaft speed as one of the inputs in engine miss-fire monitoring.

The camshaft position (CMP) sensor is located in the distributor and consists of a signal generator (a hall element (3) and magnet (2) ) and a signal rotor (1). When the signal rotor turns, a magnetic flux from the magnet is applied to the hall element repeatedly. The hall element generates a voltage that is proportional to the magnetic flux. This voltage signal is wave-shaped and is modified into a digital pulse by the comparator and sent to the PCM. The PCM uses the 4 pulse/revolution signal in determining the engine speed and the position of each cylinder. The PCM uses the CMP sensor input in order to control the fuel injectors and the ignition timing.

The fuel level sensor is located in the fuel tank. The fuel level sensor sends a signal to the powertrain control module (PCM) and the fuel gauge in the instrument panel. The PCM uses the signal from the fuel level sensor as one of the monitoring conditions for detecting EVAP control system DTCs. The PCM also uses the fuel level sensor input in order to control the EVAP tank pressure control solenoid vacuum valve. If the fuel level is greater than a specified value, the PCM will operate the EVAP tank pressure control solenoid vacuum valve in order to prevent liquid fuel from flowing into the EVAP canister from the fuel tank.

The fuel tank pressure sensor is installed on top of the fuel tank. The fuel tank pressure sensor senses the fuel vapor pressure in the fuel tank and compares it with the barometric pressure. The PCM will then convert the pressure reading into a voltage signal. The PCM uses this signal as one of the inputs to detect an EVAP control system malfunctions.

The fuel tank pressure sensor is similar to the Manifold Absolute Pressure (MAP) sensor. The fuel tank pressure sensor measures the difference between the air pressure (or vacuum) in the fuel tank and the atmospheric pressure. The PCM supplies a 5 volt reference and a ground to the sensor. The sensor sends a voltage signal between 0.1 and 4.9 volts back to the PCM. When the fuel cap is removed, the air pressure in the fuel tank is equal to the atmospheric pressure and the output voltage of the sensor will range from 2.0 to 2.5 volts.

This signal is sent from the engine starter circuit. Once the signal is received, the powertrain control module (PCM) detects that the engine is cranking and will use the PCM as one of the signals to control the fuel injection timing and the idle air control (IAC) valve operation.

This signal is sent to the powertrain control module (PCM) from the Park/Neutral position (PNP) switch whenever the manual selector lever is in the P or N position (automatic transmission models only). The PCM uses this signal as one of the inputs to control fuel injection timing and idle air control (IAC) valve operation.

The clutch pedal position switch (CPP) is closed when the clutch pedal is depressed. The powertrain control module receives a voltage signal from the CPP switch when the CPP switch is closed and the ignition switch is in the Start (crank) position. The starter motor solenoid also receives an input from the CPP switch.

The A/C compressor control module provides an A/C idle-up signal to the powertrain control module (PCM) when the A/C compressor clutch is in operation. The PCM uses the A/C idle-up signal in order to modify the engine idle speed. The PCM will signal the idle air control (IAC) valve to open the air passage in the throttle body. Opening the IAC air passage slightly will increase the engine speed in order to prevent a rough idle or a stalling condition.

The power steering pressure (PSP) switch signals the powertrain control module (PCM) when the vehicle is in need of power steering assist. The turning of the steering wheel causes increased power steering fluid pressure which will close the PSP switch. The PCM uses this signal to tell the idle air control (IAC) valve to raise the idle speed before the load can cause a poor idling condition. The PSP switch is located in the power steering pump housing.

The closed throttle position (CTP) switch detects whether or not the throttle plate is at a closed (idle) position. The CTP switch sends an ON signal to the PCM when the the throttle plate is closed. When the throttle plate is open beyond a specified value the CTP switch will send an OFF signal to the PCM.

The CPT switch consists of a hall switch integrated circuit, a magnet, and a spring. The CTP switch is located inside of the throttle position sensor. In order to service the CTP switch the throttle position sensor must be replaced.