P0151

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 0-1000 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 low voltage for more than a specified number of seconds during the test conditions.

CONDITIONS FOR RUNNING THE DTC
Lean Test Enable
^ DTCs P0101, 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 or 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 fuel system is operating in Closed Loop.
^ The accelerator pedal position (APP) indicated angle is between 3-70 percent.
^ The fuel level is more than 10 percent.

or

Power Enrichment Test Enable
^ DTCs P0101, 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, or P1518 are not set.
^ The AIR and the EGR diagnostics are not active.
^ The ignition voltage is between 11-18 volts.
^ The fuel system is operating in Closed Loop.
^ The fuel level is above 10 percent.
^ The power enrichment (PE) mode is enabled for more than 1 seconds.

CONDITIONS FOR SETTING THE DTC
Lean Test
^ The HO2S signal voltage remains below 200 mV.
^ The Criteria 1 conditions are present for 155 seconds.

or

Power Enrichment Test
^ The HO2S signal voltage remains below 360 mV.
^ The Criteria 2 conditions are present for 10 seconds during the PE mode.

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 1-7:

Steps 8-14:

The numbers below refer to the step numbers on the diagnostic table.
4. This step tests for a shorted HO2S high signal circuit.
5. This step tests for an open HO2S low reference circuit.
8. Monitor the HO2S voltage of the opposite bank sensor. If the voltage activity of the opposite bank sensor is similar to the voltage activity of the suspect sensor, test for lean conditions that would affect both cylinder banks. An opposite bank sensor with normal HO2S voltage activity indicates the suspect HO2S needs replacement or a lean condition exists only on the suspect HO2S cylinder bank.