P0126

DTC P0126

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle (Initial Inspection and Diagnostic Overview) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Initial Inspection and Diagnostic Overview) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Initial Inspection and Diagnostic Overview) provides an overview of each diagnostic category.

DTC Descriptors
DTC P0126
- Engine Coolant Temperature (ECT) Insufficient for Closed Loop

Circuit Description

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.

Conditions for Running the DTC

* DTCs P0116, P0117, P0118, and P0217 are not set.
* The engine is running.
* The engine has been at an idle state for 100-600 seconds.
* The startup engine coolant temperature is less than 34°C (93°F).
* The DTC runs continuously when the above conditions are met.

Conditions for Setting the DTC

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.

Action Taken When the DTC Sets

DTC P0126 is a Type B DTC.

Conditions for Clearing the MIL/DTC

DTC P0126 is a Type B DTC.

Diagnostic Aids

If the customer's concern is that the engine does not reach operating temperature or that the heater output is low.

Reference Information
Schematic Reference

Engine Controls Schematics (Electrical Diagrams)
Electrical Information Reference

* Circuit Testing (Component Tests and General Diagnostics)
* Connector Repairs (Component Tests and General Diagnostics)
* Testing for Intermittent Conditions and Poor Connections (Component Tests and General Diagnostics)
* Wiring Repairs (Component Tests and General Diagnostics)

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions (Diagnostic Trouble Code Descriptions)
Scan Tool Reference

Control Module References (Programming and Relearning) for scan tool information

Circuit/System Verification

1. Perform the Loss of Coolant (Loss of Coolant) to verify correct fluid level.
2. Engine running, observe the DTC information with a scan tool. DTC P0126 should not set.
3. 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.

Circuit/System Testing

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.

Component Testing

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 (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.

Repair Instructions

Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.

* Engine Coolant Temperature Sensor Replacement (Service and Repair)
* Control Module References (Programming and Relearning) for ECM replacement, setup, and programming