P0161

Schematic:

CIRCUIT DESCRIPTION
The Heated Oxygen Sensor (HO2S) heater circuit reduces the time required for the oxygen sensor to reach operating temperature and maintains the operating temperature during extended idle periods. When the ignition is turned to the ON position, voltage is supplied to the sensor heater from the engine controls power relay through a fuse. The Engine Control Module (ECM) controls the heater operation by first modulating the ground circuit when the sensors are cold, and then turning the heaters ON continuously after a predetermined amount of time. This prevents the possibility of thermal shock to the sensor by controlling the sensors rate of heating. Once the sensor reaches operating temperature, the ECM may again modulate the ground.

CONDITIONS FOR RUNNING THE DTC
^ The battery voltage is between 11 and 15.5 volts.
^ The catalytic convertor temperature is between 330°C and 520°C (626°F and 968°F).
^ There are no rear HO2S heater Diagnostic Trouble Codes (DTCs) present.

CONDITIONS FOR SETTING THE DTC
The ECM has calculated that the HO2S heater resistance is out of range for 15 seconds.

ACTION TAKEN WHEN THE DTC SETS
The ECM illuminates the Malfunction Indicator Lamp (MIL) and records the operating conditions in the Freeze Frame on the second consecutive drive trip that the diagnostic runs and fails.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The ECM turns OFF the MIL after three consecutive drive trips that the diagnostic runs and passes.
^ A History DTC clears after forty consecutive warm-up cycles in which no failures are reported by this diagnostic or any other emission related diagnostic.
^ The scan tool clears the MIL/DTC.

DIAGNOSTIC AIDS
Use the J 35616-A connector test adapter kit for any test that requires probing the ECM harness connector or a component harness connector.

An open fuse in the HO2S heater circuit may be caused by the heater element in one of the sensors. The condition may not be present until the sensor operates for a period of time. If no fault is present in the heater circuit, monitor the amperage of each heater using the DMM to determine if one of the heater elements is the cause of the open fuse.

Check for the following conditions:
^ The sensor pigtail or the harness may be mis-positioned and contacting the exhaust system.
^ An intermittent or poor connection or corrosion in the connector: Refer to Testing for Intermittent and Poor Connections in Diagrams.

TEST DESCRIPTION

Steps 1 - 9:

Steps 10 - 17:

Steps 18 - 27:

The numbers below refer to the step numbers on the diagnostic table.
2. Multiple HO2S DTCs will set if the supply voltage to the sensor heaters is open.
9. The test lamp may illuminate or flash depending on the operating conditions. This indicates the ECM and the control circuit are functioning. The engine must be running for the ECM to operate the control circuit.
11. The ECM provides the reference ground for the oxygen sensor. A small amount of current will appear on this circuit when the ECM is operational. The measured resistance of the circuit will increase when the ECM is operational. This is considered normal.