Circuit Description
An engine coolant temperature (ECT) sensor monitors the temperature of the coolant. The fuel burned since start up is used to determine if the engine has been driven within 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 regulating temperature of the thermostat, diagnostics that use engine coolant temperature as enabling criteria, may not run when expected.
If engine coolant fails to reach a preset target temperature before the calculated amount of fuel is burned, DTC P0128 will set.
This diagnostic trouble code (DTC) will only run once per ignition cycle until a Pass, Fail or Disable condition exists.
Conditions for Running the DTC
| • | DTCs P0112, P0113, P0117, or P0118 are not set. |
| • | The coolant temperature is more than -7°C (19°F). |
| • | The air temperature is more than -7°C (19°F). |
| • | The engine has been operating less than 1,320 seconds (22 minutes). |
Conditions for Setting the DTC
| • | The engine coolant temperature is less than 72°C (161°F). |
| • | The air temperature is more than -7°C (19°F). |
| • | The engine has been operating more than 1,320 seconds (22 minutes). |
| • | The fuel burned since start up is more than 12 million mm³. |
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 MIL/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 ECT sensor can cause a DTC P0128 to set. |
| • | This test will not run if either the intake air or coolant temperature is too low at start up. |
| • | If an intermittent condition is suspected refer to Intermittent Conditions . |
| • | Using Freeze Frame/Failure Records may aid in determining the conditions under which the failure occurred. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
This step tests for excessive resistance in the ECT circuit.
-
This step tests for a skewed sensor through the range of temperatures affecting this DTC.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics | ||||
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | |
2 | Is the cooling system low on coolant? | -- | Go to Loss of Coolant in Engine Cooling | Go to Step 3 |
Does the scan tool indicate that the ECT sensor temperature is more than the specified value? | 146°C (294°F) | Go to Step 7 | Go to Step 4 | |
Does the scan tool indicate that the ECT sensor temperature is more than the specified value? | 146°C (294°F) | Go to Step 5 | Go to Step 6 | |
5 |
Did you find and correct the condition? | -- | Go to Step 10 | -- |
6 |
Did you find and correct the condition? | -- | Go to Step 10 | -- |
7 |
Does the resistance and temperature closely match the Temperature vs Resistance Table? | -- | Go to Step 8 | Go to Step 9 |
8 | Reinstall the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement . Did you complete the action? | -- | Go to Engine Fails To Reach Normal Operating Temperature in Engine Cooling | -- |
9 | Replace the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement . Did you complete the replacement? | -- | Go to Step 10 | -- |
10 |
Does the DTC run and pass? | -- | Go to Step 11 | Go to Step 2 |
11 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK | |
