P0125

DTC P0125: Engine Coolant Temperature (ECT) Insufficient for Closed Loop Fuel Control

Wiring Diagram:

Circuit Description
When the vehicle is first started, it operates in Open Loop, ignoring the Oxygen Sensor (O2S) signal and calculating the fuel/air ratio based on inputs from the Engine Coolant Temperature (ECT), Throttle Position (TP) and Manifold Absolute Pressure (MAP) sensors only. The Engine Control Module (ECM) will begin using the O2S 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 O2S has a varying voltage output showing that it is hot enough to operate properly.
- The ECT has increased a minimum amount based on ECT at engine start up.

Conditions for Setting the DTC
- Start-up ECT is less than 9.75 degrees C (49.55 degrees F).
- Battery voltage is between 9.953 and 25.898 V.
- No ECT or IAT faults exist.

Action Taken When the DTC Sets
- The Malfunction Indicator Lamp (MIL) will illuminate.
- The ECM 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.

Conditions for Clearing the MIL/DTC
- The MIL will turn OFF at the end of 3 consecutive validation cycles in which the diagnostic runs without a fault.
- A history DTC will clear after 40 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 a poor connection or damaged ECM harness. Inspect the ECT sensor signal circuit and ground circuit terminals 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 ECT sensor signal circuit and ground circuit terminals 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 to evaluate the possibility of a skewed sensor.

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DTC Troubleshooting