P0140

CIRCUIT DESCRIPTION
The Powertrain Control Module (PCM) supplies a bias voltage of about 450 mV between the HO2S signal circuit and the HO2S low signal circuit. When measured with a 10 megohm digital voltmeter, this voltage may display as low as 350 mV. The oxygen sensor signal varies from about 800 mV when the exhaust is rich, to about 50 mV when the exhaust is lean. The PCM constantly monitors the HO2S signal during closed loop operation. The PCM then compensates for a rich or lean condition by decreasing or increasing the injector pulse width as necessary. If the HO2S 2 voltage remains at or near the 450 mV bias for an extended period of time, DTC P0140 will set.

CONDITIONS FOR RUNNING THE DTC
^ DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0128, P0201, P0202- P0206, P0410, P0440, P0442, P0443, P0446, P0449, or P1441 are not set.
^ The engine has been running for at least 4 minutes.

CONDITIONS FOR SETTING THE DTC
^ The HO2S 2 signal voltage remains between 412-500 mV for up to 90 seconds.
^ The ignition voltage should be more than 9 volts and less than 18 volts
^ The test only runs once per trip.
^ A Throttle Position (TP) sensor change of more than 8 percent occurs at least twice during the test.

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.

TEST DESCRIPTION

Steps 1-6:

Steps 7-13:

Steps 14-18:

The numbers below refer to the step numbers on the diagnostic table.
2. This step checks for the correct sensor activity. When in Closed Loop fuel control, the HO2S voltage should rapidly swing above and below the bias voltage.
4. This step checks the PCM and the high and low signal circuits between the PCM and the HO2S connector for correct operation.
7. This step checks for correct HO2S heater circuit operation up to the HO2S connector.
8. This step checks for correct circuit resistance between the HO2S low signal circuits and the PCM ground.
13. This step checks for proper circuit resistance between the HO2S low signal circuits and the PCM ground with a COLD sensor. A loose HO2S or poor thread to exhaust electrical contact will cause higher resistance when the HO2S is cold. Although 500 ohm is allowed, typical resistance should be less than 50 ohm.