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 10°C (50°F) and 20°C (68°F). |
| • | The intake air temperature (IAT) is between -9 to +36°C (14-97°F). |
| • | DTC P0128 runs once per drive cycle when the above conditions have been met. |
Conditions for Setting the DTC
The engine coolant temperature failed to reach at least 75°C (167°F) during the running of the water temperature counter diagnostic.
Action Taken When the DTC Sets
| • | The PCM illuminates the malfunction indicator lamp (MIL). |
| • | 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. Refer to Thermostat Diagnosis in Engine Cooling. |
| • | An incorrect thermostat can cause the engine to warm up slowly. Replace any thermostat that does not meet OEM specifications. Refer to Thermostat Diagnosis in Engine Cooling. |
| • | 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 Versus 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
The numbers below refer to the step numbers on the diagnostic table.
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The Diagnostic System Check-Engine Controls 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.
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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.
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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.
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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.
Step | Action | Values | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics | ||||||||||
Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | ||||||||
Are the ECT and the IAT readings within 2 or 3 degrees of each other? | -- | Go to Step 3 | Go to Step 6 | |||||||
Does the ECT indicated on the scan tool increase by more than the specified value within the 10 minutes? | 20°C (36°F) | Go to Step 5 | Go to Step 4 | |||||||
4 | Are any ECT sensor DTCs set? | -- | Go to Step 8 | |||||||
5 | The fault is not present. Are there any DTCs stored that have not been diagnosed? | -- | Go to Diagnostic Aids | |||||||
Is the resistance of the ECT sensor within specifications? | -- | Go to Step 7 | Go to Step 9 | |||||||
7 |
Did you find and correct the condition? | -- | Go to Step 11 | Go to Step 10 | ||||||
8 |
Did you complete the action? | -- | Go to Step 11 | -- | ||||||
9 | Replace the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement . Did you complete the replacement? | -- | Go to Step 11 | -- | ||||||
10 | Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | -- | Go to Step 11 | -- | ||||||
11 |
Does the DTC run and pass? | -- | Go to Step 12 | Go to Step 2 | ||||||
12 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK | |||||||
