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 coming into the engine is accumulated and used 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 diagnostics that use ECT as enabling criteria may not run when expected.
This DTC will only run once per ignition cycle within the enabling conditions. If the ECM detects the calibrated amount of air flow and engine run time have been met, and the ECT has not met the minimum thermostat regulating 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 P0101, P0102, P0103, P0112, P0113, P0117, P0118, P0502, P0503 are not set. |
| • | The startup ECT is less than 80°C (176°F). |
| • | The intake air temperature (IAT) is more than -7°C (+19°F). |
| • | The engine run time is between 30 seconds and 30 minutes. |
| • | The vehicle is driven more than 0.8 kilometers (0.5 miles) at more than 8 km/h (5 mph). |
| • | The mass air flow (MAF) is between 20-75 g/s, with the average more than 30 g/s. |
| • | This DTC runs once per ignition cycle within the enabling conditions. |
Conditions for Setting the DTC
The ECM detects that:
| • | The calibrated amount of engine run time has been met. |
| • | The calibrated amount of engine air flow has been met. |
| • | The calibrated vehicle speed and distance have been met. |
| • | The calibrated ECT of 80°C (176°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. |
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 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 | Measure the voltage from the signal circuit of the ECT sensor to a good ground with a DMM. Refer to Circuit Testing in Wiring Systems. Is the voltage within the specified range? | 4.9-5.1 V | Go to Step 6 | Go to Step 8 | ||||||
6 | Measure the voltage from the signal circuit of the ECT sensor to the low reference circuit of the ECT sensor with a DMM. Refer to Circuit Testing in Wiring Systems. Is the voltage within the specified range? | 4.9-5.1 V | Go to Step 9 | Go to Step 7 | ||||||
7 | Test the ECT sensor low reference circuit for high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 11 | ||||||
8 | Test the ECT sensor signal circuit for high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 11 | ||||||
9 |
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 10 | Go to Step 12 | ||||||
10 | Install the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement . Is the action complete? | -- | Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems | -- | ||||||
11 | Test for an intermittent and for a poor connection at the engine control module (ECM). Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 13 | ||||||
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 | ||||||
