P0128

DTC P0128

DTC DESCRIPTORS

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

DTC P0128
Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature

DIAGNOSTIC FAULT INFORMATION
Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. Testing and Inspection

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. If the engine coolant temperature fails to reach the closed loop temperature or the thermostat regulating temperature, before a predetermined amount of air flow enters the engine, the applicable DTC sets.

CONDITIONS FOR RUNNING THE DTC

P0125
- 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).
- This DTC runs continuously when the above conditions are met.

P0128
- DTCs P0116, P0117, P0118, and P0217 are not set.
- The engine is running.
- The intake air temperature (IAT) sensor is greater than -39°C (-38°F).
- This DTC runs continuously when the above conditions are met.

CONDITIONS FOR SETTING THE DTC

P0125
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.

P0128
The ECM detects that the amount of time it takes for the ECT to reach maximum temperature has exceeded a calibrated time.

ACTION TAKEN WHEN THE DTC SETS
DTC P0125 and P0128 are Type B DTCs.

CONDITIONS FOR CLEARING THE MIL/DTC
DTC P0125 and P0128 are Type B DTCs.

DIAGNOSTIC AIDS
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. Symptom Related Diagnostic Procedures

CIRCUIT/SYSTEM VERIFICATION
1. Verify that the engine coolant level is correct.
- If the engine coolant is not at the correct level, refer to Loss of Coolant. Loss of Coolant
2. Verify that the engine coolant temperature is at the normal operating temperature.
- If the engine coolant temperature is below normal operating temperature, refer to Thermostat Diagnosis. Symptom Related Diagnostic Procedures
3. Operate the vehicle within the conditions for running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records. DTC P0125 or P0128 should not set.
4. If the vehicle passes the Circuit/System Verification Test, then operate the vehicle within the conditions for running the DTC. You may also operate the vehicle within the conditions that are captured in the Freeze Frame/Failure Records List.

CIRCUIT/SYSTEM TESTING
1. Ignition OFF, disconnect the harness connector at the ECT sensor.
2. Ignition OFF, test for less than 5 ohms of resistance between the low reference circuit and ground.
- If greater than 5 ohms, test the low reference circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM.
3. Ignition ON, using a scan tool verify that the ECT sensor parameter is at -40°C (-40°F).
- If greater than -40°C (-40°F), test the signal circuit of the ECT sensor for a short to ground. If the circuit tests normal, replace the ECM.
4. Install a 3 amp fused jumper wire between the signal circuit and the low reference circuit. Verify the ECT sensor parameter is greater than 128°C (262°F).

IMPORTANT: If the signal circuit of the ECT sensor is shorted to voltage the ECT sensor may be damaged.

- If less than 128°C (262°F), test the signal circuit of the ECT sensor for a short to voltage or an open/high resistance. If the circuit test normal, replace the ECM.
5. If all circuits test normal, test or replace the ECT sensor.

COMPONENT TESTING
1. Ignition OFF, disconnect the harness connector of the ECT sensor.
2. Test the ECT sensor by varying the sensor temperature while monitoring the sensor resistance. Compare the readings with the Temperature vs Resistance table and verify that the resistance is within 5 percent of the specification.

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

- If the resistance vs. temperature is not within 5 percent, replace the ECT sensor.

REPAIR INSTRUCTIONS
Perform the Diagnostic Repair Verification after completing the diagnostic procedure. Verification Tests