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 engine coolant temperature 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 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, P0103, P0112, P0113, P0116, P0117, P0118, P0125, P0502, P0503, P1114, or P1115 are not present. |
| • | The coolant temperature is more than -40°C (-40°F). |
| • | The intake air temperature is more than -7°C (19°F). |
| • | The engine is running more than 4 minutes. |
| • | The vehicle speed sensor (VSS) average is more than 24 km/h (15 mph) over the key cycle. |
| • | The mass air flow (MAF) average reading is more than 23 g/s. |
| • | The vehicle is driven more than 5 km (3 miles). |
Conditions for Setting the DTC
The length of time for the engine coolant to reach a preset temperature is more than the time length should be.
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 will set if the diagnostic does not pass within 25 minutes after start up. |
| • | DTC P0128 is designed to detect a faulty thermostat. |
| • | It is possible for a skewed ECT sensor to cause a DTC P0128 to set. |
| • | A skewed intake air temperature (IAT) sensor may cause this DTC 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 | ||||
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | |
2 |
Important: The powertrain control module (PCM) will enable the engine cooling fans when certain engine coolant temperature (ECT) diagnostic trouble codes are set. 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 |
Did you find and correct the condition? | -- | Go to Step 10 | -- |
6 |
Did you find and correct the condition? | -- | Go to Step 10 | -- |
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 | Reinstall 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 |
Does the DTC run and pass? | -- | Go to Step 11 | Go to Diagnostic Aids |
11 | Observe the stored information, Capture Info, with a scan tool. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK | |
