P0143

Circuit Description

The PCM provides a 0.45 volt reference signal to the oxygen sensor on CKT 1668. When the oxygen sensor is cold, below 200°C (392°F), the Oxygen Sensor signal voltage will be around 0.45 volt and the PCM will keep the system in Open Loop operation. When the oxygen sensor is warm, above 200°C (392°F), the oxygen sensor will swing from rich to lean rapidly, if the PCM is in good control of the air fuel mixture. DTC P0143 is designed so that if the oxygen sensor stays at a lean voltage for 100 out of 120 seconds during the test conditions, this DTC will set. DTC P0143 will set when:

^ There is an oxygen sensor circuit fault giving a false lean indication.
^ The air fuel ratio is actually lean due to a vacuum leak or fuel control system fault.

Conditions for Running the DTC

^ DTCs P0101, P0102, P0103, P0107, P0108, P0117, P0118, P0121, P0122, P0123, P0131, P0132, P0133, P0134, P0151, P0152, P0153, P0154, P0300, P1133, P1134, P1153 and P1154 not set.
^ TP sensor between 5.4 and 25 degrees.
^ Coolant temperature 75°C (167°F) or greater.
^ Closed loop fuel control enabled.
^ Oxygen sensors ready.
^ Engine speed 800 RPM or greater.
^ MAP sensor greater than 32 kPa.
^ Above conditions met for 5 seconds.

Conditions for Setting the DTC

Oxygen sensor has been lean (49 mV (0.049 V) or less) for 100 out of 120 seconds.

Action Taken When the DTC Sets

^ The PCM will illuminate the Malfunction Indicator Lamp (MIL) when the diagnostic runs and fails.
^ The PCM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame and Failure Records.

Conditions for Clearing the MIL/DTC

^ The PCM will turn the MIL OFF after three consecutive drive trips that the diagnostic runs and does not fail.
^ A Last Test Failed (current) DTC will clear when the diagnostic runs and does not fail.
^ A History DTC will clear after forty consecutive warm-up cycles with no failures of any emission related diagnostic test.
^ Use a scan tool to clear DTCs.
^ Interrupting PCM battery voltage may or may not clear DTCs. This practice is not recommended.

Diagnostic Aids

Notice: Do not solder heated oxygen sensor wires. Soldering the wires will result in the loss of the air reference to the sensor. Check snapshot data for front and rear long term fuel trim values. If both banks were lean, check for a condition that would cause lean engine operation

Test Description

Number(s) below refer to the step number(s) on the Diagnostic Table.

2. If DTC P0146 is also set, diagnose it first because an oxygen sensor not ready can cause other emission related problems.
3. If the oxygen sensor is operating correctly, voltage will fluctuate above 0.049 volt. A lean sensor/condition will cause the voltage to remain below 0.049 volt.