P0155

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 11-15.5 volts.
^ The engine is running.
^ The sensor heater is ON.

CONDITIONS FOR SETTING THE DTC
The heater voltage is more than 3.6 volts, or less than 2.3 volts for more than 50 ms (0.05 seconds).

OR

No system readiness is detected after 20 seconds of operation on a start occurring more than 4 minutes after the engine was last run.

ACTION TAKEN WHEN THE DTC SETS
^ The control module illuminates the Malfunction Indicator Lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
^ The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The control module turns OFF the Malfunction Indicator Lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
^ A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
^ A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
^ Use a scan tool in order to clear the MIL and the 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 1-11:

Steps 12-24:

Steps 25-28:

The numbers below refer to the step numbers on the diagnostic table.
2. Multiple Diagnostic Trouble Codes (DTCs) will set if the supply voltage to the sensor heaters is open.
4. Any exhaust leak, even a small one, can affect the operation of the HO2S. The ECM compares the reading of sensor 1 to the reading of sensor 2 as part of the diagnosis. A leak between sensor 1 and sensor 2 may set a DTC.
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.
14. A short between the signal circuit and the reference ground circuit or the pump current input and output circuits may not appear on the voltage tests. The ECM must be disconnected and each circuit tested for continuity with the other HO2S circuits.