For 1990-2009 cars

### Circuit Description

An engine coolant temperature (ECT) sensor monitors the temperature of the coolant. This input is used by the powertrain control module (PCM) for engine control and as an enabling criteria for some diagnostics.

The air flow coming into the engine is accumulated and used to determine if the vehicle has been driven within the conditions that would allow the engine coolant to heat up normally to the thermostat regulating temperature. If the coolant temperature does not increase normally or does not reach the regulating temperature of the thermostat, the diagnostics that use ECT as enabling criteria may not run when expected.

If the PCM detects the calibrated amount of air flow and engine run time have been met, and the ECT has not met the minimum thermostat regulating temperature, DTC P0128 sets.

### DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0128 Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature

### Conditions for Running the DTC

• | DTCs P0101, P0102, P0103, P0112, P0113, P0117, P0118, P0502, P0503 are not set.

• | The startup ECT is less than 80°C (176°F).

• | The intake air temperature (IAT) is more than -7°C (+19°F).

• | The engine run time is between 30 seconds and 30 minutes.

• | The vehicle is driven more than 0.8 kilometers (0.5 miles) at more than 8 km/h (5 mph).

• | The mass air flow (MAF) is between 20-75 g/s, with the average more than 30 g/s.

• | This DTC runs once per ignition cycle when the above conditions are met.

### Conditions for Setting the DTC

The PCM detects that:

• | The calibrated amount of engine run time has been met.

• | The calibrated amount of engine air flow has been met.

• | The calibrated vehicle speed and distance have been met.

• | The calibrated ECT of 80°C (176°F) has not been met.

### Action Taken When the DTC Sets

• | The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.

• | The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

### Conditions for Clearing the MIL/DTC

• | The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.

• | A current DTC, Last Test Failed, clears when the diagnostic runs and passes.

• | A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.

• | Clear the MIL and the DTC with a scan tool.

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
Schematic Reference: Engine Controls Schematics Connector End View Reference: Engine Controls Connector End Views or Powertrain Control Module Connector End Views
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle
2 | Important: The cooling fans are commanded ON when certain engine coolant temperature (ECT) DTCs are set. Is the cooling system coolant low? | -- | Go to Cooling System Draining and Filling | Go to Step 3
3 | Test and verify the proper operation of the thermostat. Refer to Thermostat Diagnosis . Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 4
4 |

1. Disconnect the ECT sensor.
2. Inspect for the following conditions:

• | Engine coolant leaking through the ECT sensor

• | Corrosion on the ECT sensor terminals

• | Improper or corroded terminals at the ECT harness connector

• | Loose terminals in the ECT harness connector

• | Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

--

Go to Step 14

Go to Step 5

5 | Test for an intermittent and for a poor connection at the powertrain control module (PCM). Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 6
6 | Measure the resistance of the ECT sensor signal circuit between the sensor and the PCM with a DMM. Refer to Circuit Testing . Is the resistance within the specified range? | 0-10 ohms | Go to Step 7 | Go to Step 10 a
7 | Measure the resistance of the ECT sensor low reference circuit between the sensor and the PCM with a DMM. Refer to Circuit Testing . Is the resistance within the specified range? | 0-10 ohms | Go to Step 8 | Go to Step 11
8 |

1. Turn OFF the ignition.
2. Remove the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement .
3. Place the sensor on a work surface away from any heat source.
4. Allow the sensor to reach the ambient air temperature for 30-60 minutes.
5. Observe and record the ambient air temperature of the vehicle environment using an accurate thermometer.
6. Measure the resistance of the ECT sensor and record the value.
7. Compare the resistance measurement of the ECT sensor to the ambient air temperature on the Temperature vs. Resistance table. Refer to Temperature Versus Resistance .

Is the resistance measurement of the ECT sensor within the specified range?

--

Go to Step 9

Go to Step 12

9 | Install the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement . Is the action complete? | -- | Go to Step 13 | --
10 | Repair the high resistance in the ECT sensor signal circuit . Refer to Wiring Repairs . Did you complete the repair? | -- | Go to Step 14 | --
11 | Repair the high resistance in the ECT sensor low reference circuit. Refer to Wiring Repairs . Did you complete the repair? | -- | Go to Step 14 | --
12 | Replace the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement . Did you complete the replacement? | -- | Go to Step 14 | --
13 | Replace the PCM. Refer to Control Module References for replacement, setup and programming. Did you complete the replacement? |  | Go to Step 14 | --
14 |

1. Clear the DTCs with a scan tool.
2. Turn OFF the ignition for 90 seconds.
3. Start the engine.
4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

--

Go to Step 2

Go to Step 15

15 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List - Vehicle | System OK