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 other diagnostics. This input also determines if the ECT is insufficient for thermostat regulating of the coolant.
This engine coolant thermostat monitoring diagnostic is designed to detect a stuck open thermostat. When the engine is started the ECM uses a timer and a modeled ECT table that is based on engine coolant temperature at start-up, intake air temperature (IAT), engine speed (RPM) and engine load to determine when the calibrated threshold of 90°C (194°F) has been met. When this threshold is met, the modeled ECT is compared to the actual measured ECT.
If the ECM detects that the modeled ECT has met the threshold of 90°C (194°F) and the measured ECT has not met the minimum calibrated thermostat regulating temperature of 72°C (161°F) for more than 10 seconds, DTC P0128 sets.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P0128 Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature
Conditions for Running the DTC
| • | DTCs P0068, P0101, P0102, P0103, P0112, P0113, P0116, P0117, P0118, P0125, P1101, P1258 are not set. |
| • | The start-up ECT Sensor parameter is less than 42°C (107°F) |
| • | The Intake Air Temperature Sensor parameter is less than -10°C (+14°F). |
| • | The engine idle time is less than 95 percent. |
| • | The fuel cut off time is less than 50 percent. |
| • | The engine metal over temperature protection (EMOP) is not active. |
| • | This diagnostic will run once in a 500 ms loop, and complete in a drive cycle. |
Conditions for Setting the DTC
If the ECM detects that the modeled ECT has met the threshold of 90°C (194°F) and the measured ECT has not met the minimum calibrated thermostat regulating temperature of 72°C (161°F) for more than 10 seconds, or the ECT remains below 62°C (143°F), DTC P0128 sets.
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. |
Step | Action | Values | Yes | No | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics Connector End View Reference: Engine Controls Connector End Views or Engine Control Module Connector End Views | ||||||||||||||
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information | ||||||||||
2 |
Important: The cooling fans are commanded ON, if equipped, when certain engine coolant temperature (ECT) DTCs are set. Is the cooling system coolant low? | -- | Go to Cooling System Draining and Filling in Engine Cooling | Go to Step 3 | ||||||||||
3 | Test and verify the proper operation of the thermostat. Refer to Thermostat Diagnosis in Engine Cooling. Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 4 | ||||||||||
4 |
Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 5 | ||||||||||
5 | Test for an intermittent and for a poor connection at the engine control module (ECM). Refer to Testing for Intermittent Conditions and Poor Connections , Repairing Connector Terminals and Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 6 | ||||||||||
6 | Measure the resistance of the ECT sensor signal circuit between the sensor and the ECM with a DMM. Refer to Circuit Testing in Wiring Systems. Is the resistance within the specified range? | 0-10 ohms | Go to Step 7 | Go to Step 10 | ||||||||||
7 | Measure the resistance of the ECT sensor low reference circuit between the sensor and the ECM with a DMM. Refer to Circuit Testing in Wiring Systems. Is the resistance within the specified range? | 0-10 ohms | Go to Step 8 | Go to Step 11 | ||||||||||
8 |
Important: Do not hold the ECT sensor by the probe. Is the resistance measurement of the ECT sensor within the specified range for that temperature? | -- | Go to Step 9 | Go to Step 12 | ||||||||||
9 | Install the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement . Is the action complete? | -- | Go to Step 13 | -- | ||||||||||
10 | Repair the high resistance in the ECT sensor signal circuit. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 14 | -- | ||||||||||
11 | Repair the high resistance in the ECT sensor low reference circuit. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 14 | -- | ||||||||||
12 | Replace the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement . Did you complete the replacement? | -- | Go to Step 14 | -- | ||||||||||
13 | Replace the ECM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming. Did you complete the replacement? | -- | Go to Step 14 | -- | ||||||||||
14 |
Did the DTC fail this ignition? | -- | Go to Step 2 | Go to Step 15 | ||||||||||
15 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information | System OK | ||||||||||
