P0128

DTC P0128

DTC DESCRIPTOR

DTC P0128
Engine Coolant Temperature (ECT) Below Thermostat Temperature

DIAGNOSTIC FAULT INFORMATION






IMPORTANT: Always perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

TYPICAL SCAN TOOL DATA

ECT Sensor Temperature:

CIRCUIT/SYSTEM DESCRIPTION
The engine control module (ECM) uses this diagnostic to determine if the engine coolant temperature (ECT) has reached the minimum calibrated thermostat regulating temperature. The ECM supplies 5 volts to the ECT signal circuit , and a ground for the ECT low reference circuit. The ECM monitors the temperature of the coolant by using the input received from the ECT sensor. The ECM calculates the amount of fuel burned since start-up 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 engine is considered not warm enough for stable, low emission operation. Additionally, other diagnostics that use engine coolant temperature as enabling criteria may not run when expected.

The purpose of both these rationality diagnostics is to use the ECT sensor to determine if the engine coolant will heat up at the correct rate, and also meet the calibrated target temperatures under various operating conditions.

The ECM uses the start-up ECT and the start-up intake air temperature (IAT) to begin the diagnostic calculation. The air flow into the engine is accumulated, and vehicle speed, distance and engine run time are also factored in to determine if the ECT does increase normally and reach the calibrated target temperatures.

CONDITIONS FOR RUNNING THE DTC
- DTC P0112, P0113, P0116, P0117, P0118 are not set.
- The engine is running.
- The ambient air temperature is greater than -7°C (19°F), but less than 100°C (212°F).
- The start-up engine coolant temperature is greater than -40°C (-40°F) but less than 651°C (149°F).
- DTC P0128 will only run once per ignition cycle until a Pass, Fail or Disable condition exists.

CONDITIONS FOR SETTING THE DTC
- The ambient air temperature is greater than 10°C (50°F) and the calculated coolant temperature is greater than 80°C (176°F), but the actual coolant temperature is less than 72°C (161°F).OR
- The ambient air temperature is equal to or less than 10°C (50°F) and the calculated coolant temperature is greater than 55°C (131°F), but the actual coolant temperature is less than 50°C (122°F).

ACTION TAKEN WHEN THE DTC SETS
- The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
- The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

CONDITIONS FOR CLEARING THE DTC
- The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
- A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
- A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
- Clear the MIL and the DTC with a scan tool.

DIAGNOSTIC AIDS
- DTC P0128 is designed to detect a faulty thermostat.
- A skewed low ECT sensor can cause a DTC P0128 to set.
- A thermostat that is stuck open can cause a DTC P0128 to set. A high resistance short from the signal circuit to the low reference circuit can cause this DTC to set.

CIRCUIT/SYSTEM VERIFICATION
1. If the ignition has been OFF for 10 hours or more, the ECT and the IAT should be within 6°C (11°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.
2. Inspect the ECT sensor terminals for corrosion and for engine coolant leaking through the sensor.
3. Use the scan tool to verify the proper operation of the engine cooling fans.
4. Turn OFF the ignition.
5. Inspect the cooling system surge tank for the proper engine coolant level. Refer to Cooling System Leak Testing and Draining and Filling Cooling System.
6. Verify the proper heat range and the operation of the thermostat. Refer to Thermostat Diagnosis.

CIRCUIT/SYSTEM TESTING
1. Turn OFF the ignition and all electrical accessories. Allow sufficient time for the control module to power down before taking a resistance measurement.
2. Disconnect the ECT sensor.
3. Measure for a proper value of less than 5 ohms of resistance between the ECT low reference circuit and a good ground.
- If the resistance is more than 5 ohms, test the low reference circuit for a high resistance.
- If the ECT low reference circuit tests normal, replace the ECM.
4. Disconnect the ECM.
5. Measure for a proper value of less than 5 ohms of resistance on the ECT sensor signal circuit.
- If the resistance is more than 5 ohms, repair the high resistance in the ECT signal circuit.
6. Remove and test the ECT sensor.
- If the resistance measurements of the ECT sensor are out of range, replace the sensor.
- If the ECT sensor tests normal and the DTC continues to set, replace the ECM.

COMPONENT TESTING
Measure and record the resistance of the ECT sensor at various ambient temperatures and compare those measurements to the Temperature vs. Resistance table.

REPAIR INSTRUCTIONS

IMPORTANT: Always perform the Diagnostic Repair Verification after completing the diagnostic procedure.
- Engine Coolant Temperature (ECT) Sensor Replacement
- Control Module References for ECM replacement, setup, and programming