Circuit Description
The engine control module (ECM) uses this diagnostic to determine if the engine coolant temperature (ECT) has reached the minimum calibrated thermostat regulating temperature. The ECM supplies 5 volts to the ECT signal circuit and a ground for the ECT low reference circuit. The ECM monitors the temperature of the coolant by using the input received from the ECT sensor. The ECM calculates the amount of fuel burned since start-up to determine if the vehicle has been driven within the 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 the regulating temperature of the thermostat, the engine is considered not warm enough for stable low emission operation. Additionally other diagnostics that use engine coolant temperature as enabling criteria, may not run when expected.
If the ECM detects that the calibrated amount of fuel has been burnt and the ECT has not reached the minimum calibrated temperature 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 P0112, P0113, P0116, P0117, P0118, P0201, P0202, P0203, P0204, P0205, P0206, P0207, P0208, P0611, P0612, P1223, P1226, P1229, P1232, P1235, P1238, P1241, or P1244 are not set. |
| • | The engine coolant startup temperature is less than 51°C (124°F). |
| • | The ambient air temperature is more than -7°C (+19°F). |
| • | The engine run time is more than 925 seconds. |
| • | The fuel burned since startup is more than 8.5 million cubic millimeters. |
| • | The total idle time is less than 300 seconds. |
| • | This DTC will only run once per ignition cycle until a Pass, Fail or Disable condition exists. |
| OR |
| • | DTCs P0112, P0113, P0116, P0117, P0118, P0201, P0202, P0203, P0204, P0205, P0206, P0207, P0208, P0611, P0612, P1223, P1226, P1229, P1232, P1235, P1238, P1241, or P1244 are not set. |
| • | The engine coolant startup temperature is between 51-65°C (124-149°F). |
| • | The ambient air temperature is more than -7°C (+19°F). |
| • | The engine run time is more than 500 seconds. |
| • | The fuel burned since startup is more than 5 million cubic millimeters. |
| • | The total idle time is less than 150 seconds. |
| • | DTC P0128 will only run once per ignition cycle until a Pass, Fail or Disable condition exists. |
Conditions for Setting the DTC
The ECM detects the following:
| • | The calibrated amount of engine run time has been met. |
| • | The calibrated amount of fuel has been burnt. |
| • | The calibrated ECT of 72°C (162°F) has not been met. |
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 low ECT sensor can cause a DTC P0128 to set. |
| • | If an intermittent condition is suspected refer to Inducing Intermittent Fault Conditions in Wiring Systems and Intermittent Conditions . |
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 - Engine Controls? | -- | Go to Step 2 | |||||||||
2 | Is the cooling system coolant low? | -- | Go to Cooling System Draining and Filling in Engine Cooling | Go to Step 3 | ||||||||
3 | Perform the following actions:
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? | -- | 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 Engine Control Module Replacement . 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? | -- | System OK | |||||||||
