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 diagnostic tests. The air flow coming into the engine is accumulated. This is used to determine if the engine has been driven within conditions that would allow the engine coolant to heat up normally. If the ECT does not increase normally, or does not reach regulating temperature of the thermostat, some of the diagnostic tests may not run.
If engine coolant fails to reach a preset target temperature before a calculated air flow is accumulated DTC P0128 will set.
This diagnostic trouble code (DTC) will only run once per ignition cycle within the enabling conditions.
Conditions for Running the DTC
| • | DTCs P0101, P0102, P0103, P0112, P0113, P0117, P0118 , P0500, P0502, P0503, P1111, P1112, P1114, or P1115 are not set. |
| • | The ECT is more than -40°C (-40°F). |
| • | The start-up coolant temperature is less than 81°C (178°F). |
| • | The intake air temperature (IAT) is -7°C (+19°F) or more. |
| • | The engine running more than 2 minutes and less than 22 minutes. |
| • | The vehicle speed is more than 8 km/h (5 mph) for more than 0.5 miles. |
| • | The mass air flow (MAF) average reading is 23 g/s or more. |
Conditions for Setting the DTC
The time for the engine coolant to reach a preset temperature has been exceeded.
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 ECT sensor may cause a DTC P0128 to set. |
| • | A skewed IAT sensor may cause this DTC to set. The IAT sensor should read near ambient temperature with the engine running. |
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? | 138°C (280°F) | Go to Step 7 | Go to Step 4 | |
4 |
Does the scan tool indicate that the ECT sensor temperature is more than the specified value? | 138°C (280°F) | Go to Step 5 | Go to Step 6 |
5 | Test the ECT sensor reference low circuit for a high resistance, or for an open. Repair the circuit as necessary. Refer to Circuit Testing and to Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 10 | -- |
6 | Test the ECT sensor signal circuit for a high resistance, or for an open. Repair the circuit as necessary. Refer to Circuit Testing and to Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 10 | -- |
Does the resistance and temperature closely match the values specified? | 10-90°C (50-194°F) | Go to Step 8 | Go to Step 9 | |
8 | Reinstall the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement . Is the action complete? | -- | 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 | Use a scan tool in order to observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK | |
