GM Service Manual Online
For 1990-2009 cars only

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 Descriptor

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

Diagnostic Fault Information

Circuit

Short to Ground

High Resistance

Open

Short to Voltage

Signal Performance

ECT Sensor Signal

P0117

P0118

P0118

P0118

P0116 P0128

Low Reference

--

P0118

P0118

P0118

P0128

Typical Scan Tool Data

ECT Sensor Temperature

Circuit

Normal Range

Short to Ground

Open

Short to Voltage

ECT Sensor Signal

-39°C to +120°C (-38°F to +248°F)

150°C (303°F)

-40°C (-40°F)

-40°C (-40°F)

Low Reference

--

--

-40°C (-40°F)

-40°C (-40°F)

Circuit/System Description

An ECT sensor monitors the temperature of the coolant. This input is used by the control module for engine control and as an enabling criteria for some diagnostics. The air flow coming into the engine is accumulated and used to determine if the vehicle has been driven within the conditions that would allow the engine coolant to heat up normally to the thermostat regulating temperature. If the coolant temperature does not increase normally or does not reach the regulating temperature of the thermostat, the diagnostics that use ECT as enabling criteria may not run when expected. If the control module detects the calibrated amount of air flow and engine run time have been met, and the ECT has not met the minimum thermostat regulating temperature, DTC P0128 sets.

Conditions for Running the DTC

    • DTCs P0101, P0102, P0103, P0112, P0113, P0117, P0118, P0502, or P0503 are not present.
    • The startup ECT is less than 75°C (167°F).
    • The IAT parameter is more than -7°C (+19°F).
    • The engine run time is between 30-1,800 seconds.
    • The vehicle is driven more than 0.8 kilometer (0.5 mile) at more than 8 km/h (5 mph).
    • This diagnostic runs once per ignition cycle when the above conditions are met.

Conditions for Setting the DTC

The control module detects that the actual amount of accumulated air flow is more than the predicted amount of accumulated air flow before the ECT reaches 80°C (176°F).

Action Taken When the DTC Sets

DTC P0128 is a type B DTCs.

Conditions for Clearing the MIL/DTC

DTC P0128 is a type B DTCs.

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

    •  Engine Controls Connector End Views
    •  Powertrain Control Module 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

Circuit/System Verification

    Caution: Under pressure, the temperature of the solution in the radiator can be considerably higher, without boiling. Removing the radiator cap while the engine is hot (pressure is high), will cause the solution to boil instantaneously, with explosive force. The solution will spew out over the engine, fenders, and the person removing the cap. Serious bodily injury may result. Flammable antifreeze, such as alcohol, is not recommended for use at any time. Flammable antifreeze could cause a serious fire.

  1. Turn OFF the ignition.
  2. Inspect the cooling system surge tank for the proper coolant level. Refer to Cooling System Leak Testing and Cooling System Draining and Filling .
  3. If the ignition has been OFF for 8 hours or more, the ECT and the IAT should be within 15°C (27°F) of each other and also the ambient temperature. Turn ON the ignition, with the engine OFF, and use a scan tool to observe the IAT and the ECT sensor parameters.
  4. Use the scan tool to verify the proper operation of the engine cooling system fans.
  5. Important: A critical analysis of the operation of the thermostat is necessary to properly diagnose these DTCs.

  6. Verify the proper heat range and the operation of the thermostat. Refer to Thermostat Diagnosis .

Circuit/System Testing

    Important: All electrical components and accessories must be turned OFF and allowed to power down.

  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 terminal A and ground.
  3. 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, verify the scan tool ECT parameter is less than -39°C (-38°F).
  5. If greater than the specified range, test the signal circuit terminal B for a short to ground. If the circuit tests normal, replace the ECM.
  6. Install a 3A fused jumper wire between the signal circuit terminal B and the low reference circuit terminal A. Verify the scan tool ECT parameter is greater than 149°C (300°F).
  7. 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.
  8. If all circuits test normal, test or replace the ECT sensor.

Component Testing

Measure and record the resistance of the ECT sensor at various ambient temperatures, then compare those measurements to the Temperature vs Resistance table. Refer to Temperature Versus Resistance .

Repair Instructions

    •  Engine Coolant Temperature Sensor Replacement
    •  Control Module References for PCM replacement, setup, and programming