P0154

CIRCUIT DESCRIPTION
Heated oxygen sensors (HO2S) are used for fuel control and post catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with the oxygen content of the exhaust stream. When the vehicle is first started, the powertrain control module (PCM) operates in an Open Loop mode, ignoring the HO2S signal voltage when calculating the air-to-fuel ratio. The PCM circuitry that monitors the HO2S generates a reference, or bias, voltage of 450 mV when the sensor is cold, high resistance. Once the HO2S warms up, the HO2S generates a voltage within a range of 0-1,000 mV, depending on the exhaust gas oxygen content. High HO2S voltage output indicates a rich fuel mixture; low HO2S voltage output indicates a lean mixture. Once fueling goes Closed Loop, the HO2S voltage will fluctuate above and below the bias voltage. A heating element inside the HO2S reduces the time required for the sensor to reach operating temperature, and provide an accurate voltage signal.

This DTC is designed to detect an HO2S voltage that remains at a nominal value +/- some calibrated range for a calibrated number of seconds. This nominal value is usually the bias voltage.

CONDITIONS FOR RUNNING THE DTC
^ DTCsPO1O1, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0200, P0300, P0351 -P0358, P0400, P0401, P0404, P0405, P0410, P0440, P0442, P0443, P0446, P0449, P0452, P0453, P1120, P1125, P1220, P1221, P1258, P1404, P1441, P1514, P1515, P1516, P1517, P1518 are not set.
^ The secondary air injection (AIR) and the exhaust gas recirculation (EGR) diagnostics are not active.
^ The ignition voltage is between 11-18 volts.
^ The engine run time is more than 409 seconds.

CONDITIONS FOR SETTING THE DTC
The HO2S signal voltage is steady between 350- 550 mV for 57 seconds or more.

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.
^ Clear the MIL and the DTC with a scan tool.

TEST DESCRIPTION

Steps 1-8:

Steps 9-14:

Steps 15-22:

The numbers below refer to the step numbers on the diagnostic table.
2. After the HO2S heater is commanded ON, the HO2S heater heats up causing the HO2S signal voltage to either increase or decrease. This indicates that the HO2S heater is OK.
6. If the test lamp illuminates, the low reference circuit between the HO2S and PCM terminal C1-C3 is OK and the PCM low reference terminal is OK.
7. If the voltage is below the specified value, the high signal circuit, the low reference circuit, and the PCM are OK.
11. The opposite bank HO2S must be disconnected to isolate a short to ground in the HO2S heater high control circuit.
15. Resistance within the specified range indicates the HO2S heater is OK.