P0125

Diagnostic Chart:

Wiring Diagram:

CIRCUIT DESCRIPTION

When the vehicle is first started, it operates in Open Loop, ignoring the oxygen sensor signal and calculating the air/fuel ratio based on inputs from the Engine Coolant Temperature (ECT), throttle position and manifold absolute pressure sensors only. The PCM will begin using the oxygen sensor signal for controlling fuel delivery (Closed Loop) when the following conditions are met.

^ The engine has run a minimum amount of time based on ECT at engine start up.

^ The Oxygen Sensor (O2S 1) has a varying voltage output showing that it is hot enough to operate properly.

^ The ECT has increased a minimum amount based on the ECT at engine start up.

CONDITIONS FOR SETTING THE DTC

^ ECT is less than 60°C (140°F) for 363 seconds.

^ Intake Air Temperature (IAT) is greater than 20°C (68°F).

^ Vehicle speed is above 5 MPH.

ACTION TAKEN WHEN THE DTC SETS

^ The Malfunction Indicator Lamp (MIL) will illuminate after two consecutive ignition cycles in which the diagnostic runs with the fault active.

^ The PCM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame and Failure Records buffers.

^ A history DTC is stored.

^ Coolant fan turns ON.

CONDITIONS FOR CLEARING THE MIL/DTC

^ The MIL will turn OFF after three consecutive ignition cycles in which the diagnostic runs without a fault.

^ A history DTC will clear after 40 consecutive warm up cycles without a fault.

^ DTC(s) can be cleared by using the scan tool.

DIAGNOSTIC AIDS

When DTC P0125 is set, a skewed ECT sensor or a stuck open thermostat is indicated.

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation.

^ Check For poor connection or damaged harness. Inspect PCM harness for backed out terminals C1-08 and C2-12 for the following conditions:

- Improper mating

- Broken locks

- Improperly formed

- Damaged terminals

- Poor terminal to wire connection

- Damaged harness

^ Perform an intermittent test. If connections and harness check OK, monitor a digital voltmeter connected between PCM terminal C1-08 and C2-12 while moving related connectors and wiring harness. If a fault is induced, the resistance reading will change. This may help to isolate the location of the malfunction.

^ Use the Temperature vs. Resistance table below to evaluate the possibility of a skewed sensor.

TEST DESCRIPTION

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

1. The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

2. The engine must be allowed to cool fully before the ECT and IAT sensors will read close to the ambient temperature in order to check for a possible skewed ECT sensor.

3. Measure the engine coolant temperature with a thermometer to determine the actual value the ECT sensor should be. Take into consideration if the engine has been run and the engine coolant has been warmed without opening the thermostat.

5. This step simulates a DTC P0118. If the PCM senses the change, the PCM and wiring are OK.

6. The ECT sensor, PCM and wiring have checked OK at this point. Check for a proper thermostat and cooling fan operation.

8. Replacement PCM's must be reprogrammed. Refer to the latest Techline information for programming procedures.