P0128

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 volts 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 following conditions:
- The highest ECT required to enable other diagnostic conditions.
- A warm-up temperature within 20 °F (7 °C) of the normal operating ECT after the engine is fully warmed.
- Specifically, a failure is recorded and MIL is turned ON if the following conditions are met once per ignition cycle at cold start.
- The ECT fails to reach the predetermined temperature within the specified time, the accumulated intake airflow is above the predetermined level, and the accumulated engine idle time is below a predetermined value.

Conditions for Setting the DTC
- Engine is running.
- DTCs P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0201, P0202, P0203, P0204, P0351, P0352, P0502 and P1627 &e not set.
- Start-up ECT is less than or equal to 91 °F (33 °C).
- Ambient temperature is greater than or equal to 50 °F (10 °C), the time for ECT to reach 165 °F (74 °C) is greater than or equal to 380 - 900 seconds (based on start up ECT) and the accumulated airflow is greater than 4900 - 10500 grams (based on start up ECT) and the accumulated idle time is less than 450 seconds.
- Ambient temperature is between 20 °F (-7 °C) and 50 °F (10 °C), the time for ECT to reach 165 °F (74 °C) is greater than or equal to 1100 seconds and the accumulated airflow is greater than 12000 grams and the accumulated idle time is less than 825 seconds.

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.
- Both coolant fans turn ON.

Conditions for Clearing the MIL/DTC
- The MIL will turn OFF after three 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.
- DTC(s) can be cleared by using the scan tool.

Diagnostic Aids
After the engine starts, the ECT should rise steadily to about 194 °F (90 °C) and then stabilize when the thermostat opens. Failure of the ECT to reach 194 °F (90 °C) indicates a stuck open thermostat.

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 the connections and the harness check OK, monitor a Digital Voltmeter (DVM) connected between the ECT sensor signal circuit and ground circuit terminals while moving the related connectors and the wiring harness. If a fault is induced, the resistance reading wilt change. This may help to isolate the location of the malfunction.

Steps 1 - 4:

Steps 5 - 14:

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
1. 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.
3. The engine must be allowed to cool fully before the ECT and IAT sensors can read the approximate ambient temperature to check for a possible skewed sensor.
4. To test the thermostat, refer to Thermostat, Engine Cooling.
5. Measure the ECT with a thermometer to determine what the actual value of the ECT sensor should be.
Use the Temperature vs. Resistance table to evaluate the possibility of a skewed sensor.
7. This step simulates a DTC P0118. If the ECM senses the change, the ECM and wiring are OK.
12. The replacement ECM must be reprogrammed. Refer to the latest Techline procedure for ECM reprogramming.