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 input 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.

CONDITIONS FOR RUNNING THE DTC
- The engine coolant temperature (ECT) is between -8 to +45°C (18-113°F).
- The intake air temperature (IAT) is more than -8°C (18°F).

CONDITIONS FOR SETTING THE DTC
The engine coolant temperature failed to reach at least 75°C (167°F) with the engine running a specified period of time after engine start.

ACTION TAKEN WHEN THE DTC SETS
- The PCM illuminates the malfunction indicator lamp (MIL) after 2 consecutive ignition cycles in which the diagnostic runs with the fault active.
- The PCM records the operating conditions at the time the diagnostic fails. This information is stored in the Freeze Frame buffer.

CONDITIONS FOR CLEARING THE MIL/DTC
- The MIL turns OFF after 3 consecutively passing trips without a fault present.
- A History DTC clears after 40 consecutive warm-up cycles without a fault.
- Use the scan tool Clear DTC Information function.

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.
- 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.
- Check for the proper operation of the engine cooling system, including the correct coolant level.
- Check for a faulty electrical connection to the PCM.

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 1-2:

Steps 3-6:

Steps 7-9:

Steps 10-12:

Steps 13-16:

Steps 17-18:

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 20°C (36°F) within 10 minutes. If the temperature does not increase at least 20°C (38°F) there is a fault with the ECT sensor or the engine cooling system.
4. This step checks for ECT sensor DTCs P0116, P0117, or P0118.
6. 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.
17. Clearing the DTCs by removing the DOME fuse also clears the PCM long term memory. Using the scan tool to clear DTCs leaves the long term memory active. DTC emitting diagnostics may not run to completion as expected unless the long term memory is cleared. Removal of the DOME fuse also clears the time display of the radio. The DTC P0128 diagnostic can be monitored on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If a DTC P0128 appears in the Not Ran Since Code Cleared list, the DTC P0128 diagnostic has not yet run. When the DTC P0128 does not appear in the Not Ran Since Code Cleared list, the DTC P0128 diagnostic has run. If the MIL is NOT illuminated, and there is no Pending DTC Status in DTC Information, the DTC P0128 diagnostic has passed. The DTCs MUST be cleared in order to view the Current Status of the Not Ran Since Code Cleared list. DO NOT forget that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for Current or Pending status, in order to determine the outcome of the diagnostic test involved.