P0138

Diagnostic Chart:

Wiring Diagram:

The heated oxygen sensor (HO2S-2) consists of a zirconia electrolyte between two platinum plates. When the sensor reaches approximately 318°C (600°F) it becomes an electrical source that responds to the oxygen content in the exhaust. The PCM produces a bias voltage of approximately 450 millivolts on the oxygen sensor circuit. When the sensor is cold 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.

DTC PARAMETERS

DTC will set if the HO2S-2 voltage is greater than 1.065 volts for 100 seconds.

OR

HO2S-2 voltage is greater than 587 mV in DFCO (decel fuel cut-off) mode for 30 seconds when:

^ The system is in closed loop.
^ No other engine sensor or misfire DTCs have been set.
^ Commanded A/F ratio is approximately 14.7 to 1.

NOTE: The HO2S-2 signal is continuously checked once the above conditions have been met for 0.5 seconds.

DIAGNOSTIC AIDS

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

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

POSSIBLE CAUSES

If a front 02S DTC and a rear HO2S-2 DTC are set, diagnose the front 02S circuit first. A diagnosed front 02S DTC may lead to a mechanical or fuel system failure which set the rear HO2S-2 code.

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