P0157

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 engine control module (ECM) operates in an open loop mode, ignoring the HO2S signal voltage when calculating the air-to-fuel ratio. The ECM supplies the HO2S with a reference, or bias, voltage of about 450 mV. The HO2S generates a voltage within a range of 0-1,000 mV that fluctuates above and below bias voltage once in closed loop. High HO2S voltage output indicates a rich fuel mixture, low HO2S voltage output indicates a lean mixture. Heating elements inside the HO2S minimize the time required for the sensors to reach operating temperature, and to provide an accurate voltage signal. If the ECM detects an HO2S signal below a calibrated minimum, DTCs P0131, P0137, P0151, or P0157 will set.

Each HO2S has the following circuits:
- An HO2S high signal
- An HO2S low signal
- An HO2S heater ignition 1 voltage
- An HO2S heater low control

CONDITIONS FOR RUNNING THE DTC (FRONT SENSORS)
- DTCs P0443, P0444, or P0445 are not set.
- The engine is running.
- The calc converter temp parameter is less than 800°C (1,472°F).
- The battery voltage is more than 10.5 volts.

CONDITIONS FOR RUNNING THE DTC (REAR SENSORS)
- DTCs P0030, P0031, P0032, P0036, P0037, P0038, P0135, P0155, P0171, P0172, P0174, P0175, P0300, P0301-P0306, P0341, P0342, P0343, P0443, P0444, or P0445 are not set.
- The calc converter temp parameter is less than 800°C (1,472°F).
- The MAF parameter is more than 9.72 g/s.
- The battery voltage is more than 10.5 volts.

CONDITIONS FOR SETTING THE DTC
The DTC will set if the ECM detects an HO2S with a signal voltage below 40 mv. The condition must be present for more than 10 seconds for the front sensors, or 680 seconds for the rear sensors.

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

Step 1:

Steps 2-5:

Step 6:

Steps 7-12:

The number below refers to the step number on the diagnostic table.
6. The conditions listed in the table may contribute to the failure of the HO2S. The conditions listed apply only to this type of failure.