P0128

DTC P0128

CIRCUIT DESCRIPTION
The DTC P0128 Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature diagnostic monitors the relationship between engine temperature and engine running time. The ECT sensor signal is used to monitor the performance of the engine cooling system thermostat. A DTC P0128 may set if the ECT sensor voltage does not indicate that the engine coolant temperature has increased sufficiently during running of the diagnostic. When the ambient air temperature and engine operating conditions are right, the powertrain control module (PCM) starts the diagnostic using a water temperature counter. The water temperature counter times how long the cooling system takes to reach 75°C (167°F). Failure to reach 75°C (167°F) within the diagnostic window can indicate a malfunctioning thermostat.

DTC DESCRIPTOR
This diagnostic procedure supports the following DTC.
DTC P0128 Engine Coolant Temperature (ECT) is Below Thermostat Regulating Temperature

CONDITIONS FOR RUNNING THE DTC
- DTCs P0030, P0050, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0122, P0123, P0125, P0135, P0155, P0171, P0172, P0174, P0175, P0222, P0223, P0300-P0306, P0335, P0401, P0404, P0443, P0496, P0506, P0507, P1128, P1129, P2100, P2101, P2108, P2122, P2123, P2127, P2128, P2135, P2138, P2176, P2227, P2228, P2229, P2413, P2414, P2415, and U0107 are not set.
- The engine speed is less than 5,000 RPM.
- The engine coolant temperature (ECT) is between -7 and +35°C (20-95°F) at start-up.
- The intake air temperature (IAT) is between -7 and +35°C (20-95°F) at start-up.
- The difference between IAT temperature and ECT temperature at engine startup is less than 6°C (10°F).
- DTC P0128 runs once per drive cycle when the above conditions are met.

CONDITIONS FOR SETTING THE DTC
- The engine coolant temperature failed to reach at least 70°C (159°F).

OR

- The difference between the actual ECT temperature and the estimated coolant temperature is 10°C (18°F) or more.
- The above conditions exist when the engine has been operating for 20 minutes or less depending on the engine load.

ACTION TAKEN WHEN THE DTC SETS
- The PCM illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
- The PCM records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the PCM stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the PCM records the operating conditions at the time of the failure. The PCM writes the conditions to the Freeze Frame and updates the Failure Records.

CONDITIONS FOR CLEARING THE MIL/DTC
- The PCM turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
- A last test failed, or current DTC, clears when the diagnostic runs and does not fail.
- A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
- Use a scan tool in order to clear the MIL and the DTC.

DIAGNOSTIC AIDS
Check for the following conditions:
- A faulty thermostat that stays open slightly can cause DTC P0128 to set in cold weather when the vehicle is started and allowed to sit while warming up. Replace any suspect thermostat. Refer to Thermostat Diagnosis in Cooling System. Component Tests
- An incorrect thermostat can cause the engine to warm up slowly. Replace any thermostat that does not meet OEM Specifications.
- Use the Temperature vs Resistance table in order to evaluate the possibility of a shifted ECT sensor. A shifted sensor could result in DTC P0128. Refer to Temperature vs Resistance - Engine Coolant Temperature (ECT) Sensor.
- Check for the proper operation of the engine cooling system, including the correct coolant level.

If DTC P0128 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC was first set.

TEST DESCRIPTION

Steps 1-9:

Steps 10-18:

The numbers below refer to the step numbers on the diagnostic table.
1. The Diagnostic System Check - Computers and Control Systems prompts the technician to complete some basic checks and store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.
2. This step compares the ECT temperature to the IAT temperature. This inspection is to be performed when the vehicle has reached ambient room temperature. At that time both the surrounding air temperature and the engine coolant temperature are nearly equal. If the ECT sensor and circuit are OK, both sensors should indicate the same temperature.
3. This step checks that the temperature of the engine coolant increases at least 60°C (140°F) within 20 minutes. If the temperature does not increase to the specified value there is a fault with the ECT sensor or the engine cooling system.
4. This step checks for a correctly operating engine cooling fan. A cooling fan that comes ON too soon or stays ON all the time, can cause a DTC P0128 to set.
5. Fault not present indicates the condition that caused DTC P0128 to set is intermittent and not currently present. If no other DTCs are stored, refer to Diagnostic Aids for additional information on diagnosing an intermittent DTC P0128.
16. After replacing the PCM the relationship between the CKP sensor and the crankshaft must re-established. A new minimum throttle position and idle speed must also be established.