For 1990-2009 cars

### Circuit Description

The engine control module (ECM) monitors the temperature of the engine coolant for engine control and as an enabling criteria for some diagnostics. The amount of air flow into an engine is proportional to the amount of heat an engine generates. The ECM monitors the amount of air flow into the engine to calculate engine coolant temperature (ECT). The ECM uses the calculated temperature to determine if the engine has warmed up to the thermostat regulating temperature. If the coolant temperature does not increase normally or does not reach regulating temperature of the thermostat, diagnostics that use the ECT as enabling criteria, may not run when expected. If the engine coolant temperature fails to reach the thermostat regulating temperature, before a predetermined amount of air flow enters the engine, this DTC 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

• | Before the ECM can report DTC P0128 failed, DTC P0117 must run and pass.

• | DTCs P0101, P0102, P0103, P0112, P0113, P0118, P0722, or P0723 are not set.

• | The engine speed is more than 960 RPM.

• | The ECT is less than 71°C (160°F) at start up.

• | The calculated ambient air temperature is more than -11°C (-12°F) and less than 70°C (158°F).

• | The vehicle speed is more than 15 km/h (9 mph).

• | The air flow coming into the engine has accumulated to more than 3,000 grams.

• | The ECM detects that the engine coolant temperature rebound time is more than 5 seconds.

• | DTC P0128 runs continuously once the above conditions have been met for approximately 15 minutes.

### Conditions for Setting the DTC

• | The ECM detects that the actual coolant temperature is 10°C (18°F) lower than the calculated coolant temperature.

• | The calculated engine coolant temperature reference limit is 89.25 °C.

• | The condition exists for more than 4 seconds.

### Action Taken When the DTC Sets

• | Do not illuminate MIL.

• | The ECM records the operating conditions at the time when the diagnostic test fails. The ECM displays the failure information in the Failure Records on the scan tool.

### Conditions for Clearing the MIL/DTC

• | The DTC becomes history when the conditions for setting the DTC are no longer present.

• | The history DTC is cleared after 40 malfunction free warm-up cycles.

• | The ECM receives the clear code command from the scan tool.

### Diagnostic Aids

• | Use the J 35616-A/BT-8637 Connector Test Adapter Kit for any test that requires probing the ECM harness connector or a component harness connector.

• | The lower connector of the ECM is connector C1 and the upper connector of the ECM is connector C2. Refer to Engine Controls Component Views .

• | An engine that soaks for about 8 hours will help diagnosis the condition. After the cold soak, operate the vehicle at highway speeds for 20 minutes while monitoring the Calculated ECT - Thermostat parameter with a scan tool. If there is a condition, the calculated temperature will be 10°C (18°F) more then the actual engine coolant temperature.

• | For an intermittent condition, refer to Intermittent Conditions .

### Test Description

The numbers below refer to the step numbers on the diagnostic table.

1. This step tests for excessive resistance in the ECT circuit.

2. This step isolates the condition. If the temperature changes, test for a condition in the low reference circuit. If the temperature remains the same test the signal circuit for a condition.

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
Schematic Reference: Engine Controls Schematics Connector End View Reference: Engine Control Module Connector End Views or Engine Controls Connector End Views
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle
2 | Is the engine cooling system lack of enough coolant? | -- | Go to Loss of Coolant | Go to Step 3
3 | Is the customer's concern that the engine does not reach operating temperature or that the heater output is low? | -- | Go to Thermostat Diagnosis | Go to Step 4
4 | Caution: Refer to Road Test Caution in the Preface section.

1. Operate the vehicle within the Conditions for Running the DTC.
2. Observe the ECT sensor parameter and the Calculated ECT - Thermostat parameter with a scan tool.

Is the Calculated ECT - Thermostat parameter more than the specified value of the ECT sensor parameter?

10°C (18°F)

Go to Step 6

Go to Step 5

5 |

1. Observe the Freeze Frame/Failure Records for this DTC.
2. Turn OFF the ignition for 30 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 6

Go to Diagnostic Aids

6 | Important: Return to this diagnostic procedure after you complete the thermostat diagnosis. Test for the correct operation of the thermostat. Refer to Thermostat Diagnosis . Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 7
7 |

1. Turn OFF the ignition.
2. Disconnect the engine coolant temperature (ECT) sensor.
3. Connect a 3-amp fused jumper wire between the signal circuit and the low reference circuit of the ECT sensor.
4. Turn the ignition switch to ON position, with the engine OFF.
5. Observe the ECT sensor parameter with a scan tool.

Is the temperature more than the specified value?

139°C (282°F)

Go to Step 11

Go to Step 8

8 |

1. Connect a 3-amp fused jumper between the signal circuit of the ECT sensor and the engine control module (ECM) housing.
2. Observe the ECT sensor parameter with a scan tool.

Is the temperature more than the specified value?

139°C (282°F)

Go to Step 9

Go to Step 10

9 | Test the low reference circuit of the ECT sensor for high resistance. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 12
10 | Test the signal circuit of the ECT sensor for high resistance. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 12
11 | Test for shorted terminals and poor connections at the ECT sensor. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 13
12 | Test for shorted terminals and poor connections at the ECM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 14
13 | Replace the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement . Did you complete the replacement? | -- | Go to Step 15 | --
14 | Replace the ECM. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | -- | Go to Step 15 | --
15 |

1. Clear the DTCs with a scan tool.
2. Turn OFF the ignition for 30 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 16

16 | 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