Circuit Description
An engine coolant temperature (ECT) sensor monitors the temperature of the coolant. This input is used by the engine control module (ECM) for engine control, and as an enabling criteria for some diagnostics.
The air flow that is coming into the engine is accumulated. The accumulated air flow determines if the engine has been driven within conditions allowing the engine coolant to heat up 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 present target temperature before a calculated air flow is accumulated DTC P0128 will set.
This DTC is continuously monitored during engine operation.
Conditions for Running the DTC
| • | DTCs P0101, P0102, P0103, P0112, P0113, P0116, P0117, or P0118 are not set. |
| • | The engine block heater has not been detected or delayed for 20 seconds. |
| • | The engine off time is more than 15 minutes. |
| • | The engine speed is more than 960 RPM. |
| • | The engine coolant temperature (ECT) is less than 50°C (122°F) at start up. |
| • | The ambient air temperature is more than -39°C (-38°F) and less than 50°C (122°F). |
| • | The vehicle speed is more than 15 km/h (9 mph). |
| • | The mass air flow (MAF) average reading more than 23 g/s. |
Conditions for Setting the DTC
The time for the engine coolant to reach a set 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. |
| • | A skewed ECT sensor can cause a DTC P0128 to set. |
| • | A skewed IAT sensor can 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 that affects this DTC.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics Connector End View Reference: Engine 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 | Is the cooling system low on coolant? | -- | Go to Loss of Coolant in Engine Cooling | Go to Step 3 |
3 | Test the coolant bypass valve for proper operation. Refer to Coolant Bypass Valve Inoperative in Engine Cooling. Did you find and correct the condition? | -- | Go to Step 11 | Go to Step 4 |
Does the scan tool indicate that the ECT is more than the specified value? | 139°C (282°F) | Go to Step 8 | Go to Step 5 | |
Does the scan tool indicate that the ECT is more than the specified value? | 139°C (282°F) | Go to Step 6 | Go to Step 7 | |
6 | Test the low reference circuit for high resistance or for an open. Refer to Circuit Testing in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 11 | Go to Diagnostic Aids |
7 | Test the ECT sensor signal circuit for a high resistance or an open. Refer to Circuit Testing in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 11 | Go to Diagnostic Aids |
8 |
Does the resistance and temperature closely match the Temperature vs Resistance table? | 10- 90°C (50-194°F) | Go to Step 9 | Go to Step 10 |
9 | Install 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 | -- |
10 | Replace the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement . Did you complete the replacement? | -- | Go to Step 11 | -- |
11 |
Did the DTC fail this ignition? | -- | Go to Step 2 | Go to Step 12 |
12 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | System OK | |
