Fuel Control Heated Oxygen Sensor (HO2S 1)

HO2S 1 (Pre-catalytic Converter):

Following is a short description of how the Heated Oxygen Sensor (HO2S 1) can be diagnosed by using a scan tool. The scan tool can also be used to compare the values for a normal running engine with the engine you are diagnosing.

DESCRIPTION
The fuel control Heated Oxygen Sensor (HO2S 1) is mounted in the exhaust manifold where it 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).

DIAGNOSIS
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).

The HO2S 1 circuit, if open, should set a DTC P0134 and the scan tool will display a constant voltage between 400-500 mV. A constant voltage below 300 mV in the sensor circuit (circuit grounded) should set DTC P0131, while a constant voltage above 800 mV in the circuit should set DTC P0132.

A fault in the HO2S 1 heater circuit should cause DTC P0135 to set. The PCM can also detect HO2S response problems. If the response time of an HO2S is determined to be too slow, the PCM will store a DTC that indicates degraded HO2S performance. Refer to System DIagnosis / Diagnostic Tables / Diagnostic Trouble Code Tables. Testing and Inspection