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 5 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 start-up ECT. The Malfunction Indicator Lamp (MIL) is illuminated and DTC P0128 is set when a stuck open thermostat prevents the engine coolant from reaching the predetermined operating conditions.

Conditions for Setting the DTC
- No ECT, MAF, IAT or CKP faults exist.
- Start-up ECT is between -9.75 degrees C (14.45 degrees F) and 53.25 degrees C (127.9 degrees F).
- Short-up IAT is greater than -9.75 degrees C (14.45 degrees F).
- Measured ECT is less than 83.25 degrees C (181.9 degrees F) while modeled ECT is greater than 83.25 degrees C (181.9 degrees F).
- Measured ECT is less than 72.75 degrees C (163 degrees F) and IAT deviation is greater than -20.25 degrees C (-4.45 degrees F).

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 at the end of 3 consecutive validation cycles in which the diagnostic runs without a fault.
- A history DTC will clear after 40 warm up cycles without a fault.
- DTC(s) can be cleared by using the scan tool.

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.

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DTC Troubleshooting