For 1990-2009 cars
Engine Controls Component Views
Engine Data Sensors (1 of 2)
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice
Powertrain Control Module Connector End Views
Powertrain Control Module Connector End Views
Handling ESD Sensitive Parts Notice

### Circuit Description

The engine coolant temperature (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 is between -9°C (14°F) and 36°C (97°F).

• | The intake air temperature (IAT) is between -9°C (14°F) and 36°C (97°F).

### 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 Engine Coolant Thermostat Replacement 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 Engine Coolant Thermostat Replacement 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 engines 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 in the diagnostic table.

1. The Powertrain OBD System Check 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. 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
--- | --- | --- | --- | ---
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | Go to Step 2 | Go to Powertrain On Board Diagnostic (OBD) System Check
2 |

1. Allow the engine to cool to the ambient temperature.
2. Turn ON the ignition, with the engine OFF.
3. Compare the ECT sensor temperature reading to the IAT sensor temperature reading on a scan tool.

Are the ECT and the IAT readings within 2 or 3 degrees of each other?

--

Go to Step 3

Go to Step 6

3 |

1. Start the engine and immediately observe the ECT on the scan tool.
2. Continue to observe the ECT while idling the engine for 10 minutes.

Does the ECT indicated on the scan tool increase by more than the specified value within the 10 minutes?

20°C (68°F)

Go to Step 5

Go to Step 4

4 | Are any ECT sensor DTCs set? | -- | Go to DTC P0115 Engine Coolant Temperature (ECT) Sensor Circuit or DTC P0116 Engine Coolant Temperature (ECT) Sensor Performance | Go to Step 8
5 | The fault is not present. Are there any DTCs stored that have not been diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List | Go to Diagnostic Aids
6 |

1. Measure the resistance of the ECT sensor with a digital multimeter (DMM).
2. Compare the measured resistance of the ECT sensor with the resistance values in the Temperature vs Resistance table. Refer to Temperature Versus Resistance .

Is the resistance of the ECT sensor within specifications?

--

Go to Step 7

Go to Step 9

7 |

1. Check for any of the following conditions:

• | A high resistance in the ECT sensor wiring

• | A poor connection at the ECT sensor

• | A poor connection at the PCM

2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 11

Go to Step 10

8 |

1. Inspect the engine cooling system for all of the following:

• | The correct coolant level. Refer to Cooling System Draining and Filling in Engine Cooling.

• | The correct operation of the cooling system. Refer to Engine Fails To Reach Normal Operating Temperature in Engine Cooling.

• | The correct operation of the thermostat. Refer to Thermostat Diagnosis in Engine Cooling.

2. Repair as necessary.

Is the action complete?

--

Go to Step 11

--

9 | Replace the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement . Is the action complete? | -- | Go to Step 11 | --
10 | Replace the PCM. Refer to Powertrain Control Module Replacement . Is the action complete? | -- | Go to Step 11 | --
11 |

1. Clear the DTC Information with the scan tool.
2. Road test the vehicle within the Freeze Frame conditions that set the DTC P0128.
3. Review the scan tool data and check for DTCs.

Are any DTCs displayed on the scan tool?

--

Go to Diagnostic Trouble Code (DTC) List

System OK