P0128

DTC P0128: Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature

Wiring Diagram:

Circuit Description
The Engine Coolant Temperature (ECT) sensor uses a thermistor to control the signal voltage to the Engine Control Module (ECM). The ECM supplies a voltage on the signal circuit to the sensor. The output varies as the ECT increases. At normal engine operating temperature, the voltage will be between 1.5 and 4 V at the ECT signal terminal.

When the vehicle is first started, the ECM monitors the ECT and determines if it reaches a specified temperature level within a predetermined time. The time is based on Intake Air Temperature (IAT) and the startup ECT.

Conditions for Setting the DTC
- No disabling faults present.
- Engine is running.
- Minimum Intake Air Temperature (IAT) is greater than -7 °C (19.4 °F).
- ECT sensor reading is less than expected after the engine runs for a certain period.

Action Taken When the DTC Sets
- The Malfunction Indicator Lamp (MIL) will illuminate.
- The ECM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame and Failure Records buffers.
- A history DTC is stored.

Conditions for Clearing the MIL/DTC
- The MIL will turn OFF after four consecutive ignition cycles in which the diagnostic runs without a fault.
- A history DTC will clear after 40 consecutive warm up cycles without a fault.
- The DTC(s) can be cleared by using the scan tool.
- Disconnecting the ECM battery feed for more than 10 seconds.

Diagnostic Aids
When DTC P0128 is set, a skewed ECT sensor or a stuck-open thermostat is indicated. An intermittent may be caused by a poor connection, rubbed-through wire insulation or a wire broken inside the insulation.
Check for a poor connection or damaged ECM harness.Inspect the ECT sensor signal circuit and ground circuit terminals for the following conditions:
- Improper mating
- Broken locks
- Improperly formed
- Damaged terminals
- Poor terminal-to-wire connection
- Damaged harness

Perform an intermittent test. If connections and harness check OK, monitor a digital voltmeter connected between ECT sensor signal circuit and ground circuit terminals while moving related connectors and wiring harness. If a fault is induced, the resistance reading will change. This may help to isolate the location of the malfunction.
Use the Temperature vs. Resistance table to evaluate the possibility of a skewed sensor.

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
1. The On-Board Diagnostic (OBD II) System Check prompts the technician to complete some basic checks and store the Freeze Frame and Failure Records data on the scan tool if applicable. This creates an electronic copy of the data taken when the malfunction occurred.
The information is then stored on the scan tool for later reference.
2. The engine must be allowed to cool fully before the ECT and IAT sensors will read close to the ambient temperature in order to check for a possible skewed sensor.
3. To test the thermostat, refer to the engine cooling section.
4. Measure the engine coolant temperature with a thermometer to determine the actual value the ECT sensor should be. Take into consideration if the engine coolant has been run and the engine coolant has been warmed without opening the thermostat.
5. This step simulates a DTC P0118. If the ECM senses the change, the ECM and wiring are OK.
6. The replacement ECM must be reprogrammed.
Refer to the latest Techline procedure for ECM programming.

Step 1-5(Continued):

Step 5(Continued)-14:

Troubleshooting