The engine control module (ECM) monitors the temperature of the engine coolant for engine control and as an enabling criteria for some diagnostics. The amount of air flow into an engine is proportional to the amount of heat an engine generates. The ECM monitors the amount of air flow into the engine to calculate engine coolant temperature (ECT). The ECM uses the calculated temperature to determine if the engine has warmed up to the closed loop temperature or to the thermostat regulating temperature. If the coolant temperature does not increase normally or does not reach the closed loop temperature, diagnostics that use the ECT as enabling criteria, may not run when expected.

The ECM detects that the amount of time it takes for the ECT to reach the closed loop operating temperature has exceeded a calibrated time.

DTC P0126 is a Type B DTC.

DTC P0126 is a Type B DTC.

If the customer's concern is that the engine does not reach operating temperature or that the heater output is low, refer to Thermostat Diagnosis.

Schematic Reference

Engine Controls Schematics

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

1. Perform the Loss of Coolant to verify correct fluid level.
2. Perform the Thermostat Diagnosis to verify correct normal operating temperature.
3. Engine runnng, observe the DTC information with a scan tool. DTC P0126 should not set.
4. Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

1. Ignition OFF, disconnect the harness connector at the ECT sensor.
2. Ignition ON, clear the DTC information with a scan tool.
3. Ignition OFF, test for less than 5 ohms of resistance between the low reference circuit terminal B or 2 and ground.

⇒	If greater than the specified range, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the ECM.

4. Ignition ON, clear the DTC information with a scan tool.
5. Verify the scan tool ECT sensor parameter is at -40°C (-40°F).

⇒	If greater than the specified range, test the signal circuit for a short to ground. If the circuit tests normal, replace the ECM.

Important: If the fuse in the jumper wire opens, the signal circuit is shorted to a voltage and the sensor may be damaged.

6. Install a 1A fused jumper wire between the signal circuit terminal A or 1 and the low reference circuit terminal B or 2. Verify the scan tool ECT sensor parameter is greater than 128°C (262°F).

⇒	If less than the specified range, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM.

7. If all circuits test normal, test or replace the ECT sensor.

1. Ignition OFF, disconnect the harness connector at the ECT sensor.

Important: A thermometer can be used to the used to test the sensor off the vehicle.

2. Test the ECT sensor by varying the sensor temperature while monitoring the sensor resistance. Compare the readings with the Temperature Versus Resistance table and verify that the resistance is within 5 percent of the specification.

⇒	If not within the specified range, replace the ECT sensor.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.