For 1990-2009 cars

### Circuit Description

The rear heated oxygen (HO2S 2) sensor consists of a zirconia electrolyte between 2 platinum plates. When the reaches approximately 318°C (600°F) it becomes an electrical source that responds to the oxygen content in the exhaust. The powertrain control module (PCM) produces a bias voltage of approximately 450 millivolts on the oxygen sensor circuit. When the sensor is clod its internal resistance is extremely high, therefore, the PCM recognizes the bias as an open circuit. As the sensor warms up, the internal resistance decreases. When the sensor reaches approximately 318°C (600°F), it starts producing a voltage based on the oxygen content in the exhaust stream. Refer to DTC P0141 for heater circuit operation.

### Conditions for Setting the DTC

DTC P0140 will set if the HO2S-2 voltage is between 391-491 millivolts for 90 seconds when:

• | The engine is at normal operating temperature, the engine coolant temperature reads over 60°C (140°F).

• | The engine run time is greater than 1.0 minute.

• | No other engine sensor or misfire DTCs have been set.

• | The commanded A/F ratio is approximately 14.7 to 1.

Important: The Ho2S-2 signal is continuously checked once the above conditions have been met.

### Diagnostic Aids

If a DTC P0141, heater malfunction, is also set, repair that code first. An inoperative heater may cause the sensor to cool down, which may cause the inactive DTC to set.

The PCM will not perform a catalyst monitor test if a DTC P0140 is set.

When attempting to diagnose an intermittent problem, use the scan tool to review the malfunction history diagnostic information. The supplemental data can be used to duplicate a problem.

Intermittents or opens suspected to be at the connector can be detected by using a diagnostic service probe. Voltage can be read on wires without disconnecting any connectors.

Check the tightness of the female terminal grip with a spare male terminal.

Normal scan tool readings in Closed Loop will show the oxygen sensor voltage varying between 10 and 999 millivolts.

DTC P0140 can be set or use false oxygen readings if the oxygen sensor ground becomes loose. Refer to PCM and Engine Grounds in this manual.

Important: Do not apply grease or any material in the HO2S-2 connector or alter the wiring in any way. The wires carry reference air to the sensor used to determine the difference in oxygen from the exhaust gas to the PCM.