Circuit Description
An engine coolant temperature (ECT) sensor monitors the temperature of the coolant. This input is used by the powertrain control module (PCM) 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 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 ECT as enabling criteria, may not run when expected.
If engine coolant fails to reach a preset target temperature before a calculated air flow is accumulated DTC P0128 sets.
Conditions for Running the DTC
| • | DTCs P0101, P0102, P0103, P0112, P0113, P0116, P0117, P0118, P0125, P0500, P0502, P0503, P1111, P1112, P1114, P1115 are not present. |
| • | The start up engine coolant temperature is less than 70°C (158°F). |
| • | The intake air temperature (IAT) sensor parameter is between -7 to +55°C (19-131°F). |
| • | The engine is running between 120-1,200 seconds. |
| • | The vehicle speed is more than 5 km/h (3 mph) for more than 2.5 kilometers (1.5 miles). |
| • | The mass air flow (MAF) is between 15-75 g/s with the average more than 14 g/s. |
Conditions for Setting the DTC
| • | The calibrated amount of the engine runtime has been met. |
| • | The calibrated amount of engine air flow has been met. |
| • | The calibrated minimum engine coolant temperature of 75°C (167°F) has not been met. |
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. |
| • | Excessive coolant flow through the bypass hose to the throttle body may cause this DTC to set. |
| • | A skewed low ECT sensor can cause a P0128 to set. |
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 Connector End View Reference: Powertrain Control Module Connector End Views or Engine Controls Connector End Views | ||||
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | |
2 |
Important: The PCM will enable the engine cooling fans when certain ECT diagnostic trouble codes are set. Is the cooling system coolant low? | -- | Go to Loss of Coolant in Engine Cooling | Go to Step 3 |
Is the ECT sensor parameter more than the specified value? | 138°C (280°F) | Go to Step 7 | Go to Step 4 | |
4 |
Is the ECT sensor parameter more than the specified value? | 138°C (280°F) | Go to Step 5 | Go to Step 6 |
5 | Test the ECT sensor low reference circuit for a high resistance or an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 10 | Go to Diagnostic Aids |
6 | Test the ECT sensor signal circuit for a high resistance or an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 10 | Go to Diagnostic Aids |
Does the resistance and temperature closely match the Temperature vs Resistance Table? | 10- 90°C (50-194°F) | Go to Step 8 | Go to Step 9 | |
8 | Install the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement . Is the action complete? | -- | Go to Thermostat Diagnosis 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 |
Did the DTC fail this ignition? | -- | Go to Step 2 | Go to Step 11 |
11 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | System OK | |
