Circuit Description
The engine control module (ECM) monitors the temperature of the engine coolant for engine control and as an enabling criteria for some diagnostics. The amount of air flow into an engine is proportional to the amount of heat an engine generates. The ECM monitors the amount of air flow into the engine in order to calculate engine coolant temperature (ECT). The ECM uses the calculated temperature to determine if the engine has warmed 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 the ECT as enabling criteria, may not run when expected. If the engine coolant temperature fails to reach the thermostat regulating temperature, before a predetermined amount of air flow enters the engine, this DTC sets.
DTC Descriptors
This diagnostic procedure supports the following DTC.
DTC P0128 engine coolant temperature (ECT) below thermostat regulating temperature.
Conditions for Running the DTC
| • | DTCs P0101, P0102, P0103, P0112, P0113, P0116, P0117, P0118, P0125, P1111, P1112, P1114, or P1115 are set. |
| • | The coolant temperature at start up is less than 70°C (158°F). |
| • | The minimum air temperature is less than 54°C (129°F) and more than -7°C (19°F). |
| • | The engine has been running more than 2 minutes, but less than 22 minutes. |
| • | The vehicle speed is more than 5 mph for more than 1.5 miles. |
| • | The mass air flow average reading is more than 20 g/s and less than 75 g/s. |
| • | DTC P0128 runs once per ignition cycle when the above conditions are met. |
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. Refer to Thermostat Diagnosis in Engine Cooling. |
| • | Excessive coolant flow through the bypass hose to the throttle body may cause this DTC to set. |
| • | A skewed ECT sensor can cause a DTC P0128 to set. Refer to Temperature Versus Resistance . |
| • | 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 sensor circuit.
-
This step determines whether the high resistance is in the ECT sensor signal or ground circuit.
-
This step tests for a skewed ECT sensor.
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 | 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 | |
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 9 | -- |
6 |
Did you find and correct the condition? | -- | Go to Step 9 | -- |
Did the resistance and temperature closely match the Temperature vs Resistance Table? | -- | Go to Engine Fails To Reach Normal Operating Temperature in Engine Cooling | Go to Step 8 | |
8 | Replace the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement . Did you complete the replacement? | -- | Go to Step 9 | -- |
9 |
Does the DTC run and pass? | -- | Go to Step 10 | Go to Diagnostic Aids |
10 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK | |
