GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2006 🢒 Optra - Mexico 🢒 Restraints 🢒 SIR 🢒 DTC P0125 or P0128

Diagnostic Instructions
    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptors

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

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

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 P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0122, P0123, P0261, P0262, P0264, P0265, P0267, P0268, P0270, P0271, P0351, P0352, P0502, P0562, P0563, P0606, P2110, and U0101 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 calculated airflow into the engine is greater than a calibrated amount dependant on start-up coolant temperature.
    • The DTC runs continuously when the above conditions are met.

P0128

    • DTCs P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0261, P0261, P0264, P0265, P0267, P0268, P0270, P0271, P0335, P0336, P0351, P0352, P0562, P0563 P0606, P2110, and U0101 are not set.
    • The engine is running.
    • The intake air temperature (IAT) sensor is greater than -7°C (19°F).
    • The calculated airflow into the engine is greater than a calibrated amount dependant on start-up coolant temperature.
    • The 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 engine coolant temperature to reach the Closed Loop operating temperature has exceed a calibrated time.

P0128

The ECM detects that the amount of time it takes for the engine coolant temperature to reach maximum temperature has exceed 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 concern is that the engine does not reach operating temperature or that the heater output is low, refer to Thermostat Diagnosis .

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

    •  Engine Control Module Connector End Views
    •  Engine Controls Connector End Views

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

    •  Scan Tool Data List
    •  Scan Tool Data Definitions
    •  Scan Tool Output Controls

Circuit/System Verification
  1. Verify that the engine coolant level is correct.
    2. ⇒If the engine coolant is not at the correct level, refer to Loss of Coolant .
  2. Verify that the engine coolant temperature is at the normal operating temperature.
    3. ⇒If the engine coolant temperature is below normal operating temperature, refer to Thermostat Diagnosis .
  3. Operate the vehicle within the conditions for running the DTC. 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 Data List.

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 ON, verify that the ECT sensor parameter is at -40°C (-40°F).
    4. ⇒If greater than the specified value, test the signal circuit for a short to ground. If the circuit tests normal, replace the ECM.
  4. Test for less than 0.5 volt between the low reference circuit and ground.
    5. ⇒If greater than the specified value, test the low reference circuit for a short to voltage. If the circuit/connections test normal, replace the ECM.
  5. Ignition OFF, test for less than 5 ohms of resistance between the low reference circuit and ground.
    6. ⇒If greater than the specified value, test the low reference circuit for an open/high resistance. If the circuit/connections test normal, replace the ECM.
  6. Ignition ON, clear the DTC information with a scan tool.

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

  7. Install a 1-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).
    8. ⇒If less than the specified value, test the signal circuit for a short to voltage or an open/high resistance. If the circuit test normal, replace the ECM.
  8. 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. 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.
    3. ⇒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.

    •  Engine Coolant Temperature Sensor Replacement
    •  Control Module References for engine control module replacement, setup, and programming