For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provides an overview of each diagnostic category.

### DTC Descriptor

DTC P0128 00: Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
ECT Sensor Signal | P0117 00 | P0118 00 | P0118 00* | P0116 00, P0119 00, P0128 00
Low Reference | -- | P0118 00 | P0118 00* | P0119 00, P0128 00
*Engine control module (ECM) or sensor damage may occur if the circuit is shorted to B+

### Typical Scan Tool Data

#### ECT Sensor

Circuit | Short to Ground | Open | Short to Voltage
--- | --- | --- | ---
Operating Conditions: The ignition is ON, or the engine is running Parameter Normal Range: Varies with ambient temperature, within a range of -39 to +120°C (-38 to +248°F)
ECT Sensor Signal | 150°C (302°F) | -40°C (-40°F) | -40°C (-40°F)
Low Reference | -- | -40°C (-40°F) | -40°C (-40°F)

### Circuit/System Description

The engine coolant temperature (ECT) sensor is a variable resistor that measures the temperature of the engine coolant. The engine control module (ECM) supplies 5 V to the engine coolant temperature (ECT) sensor signal circuit and a ground for the low reference circuit. The purpose of this diagnostic is to analyze the performance of the thermostat by comparing the measured engine coolant temperature (ECT) sensor to a modeled engine coolant temperature using information from the following inputs:

• | Engine coolant temperature (ECT) sensor

• | Engine load

• | Intake air temperature (IAT)

• | Manifold absolute pressure (MAP)

• | Vehicle speed

The ECM uses the start-up ECT and the start-up IAT to begin the diagnostic calculation. The air flow into the engine is accumulated, and vehicle speed, distance, engine load, and engine run time are also factored in to determine if the ECT does increase normally and then maintains that temperature within a calibrated range of the modeled ECT.

### Conditions for Running the DTC

• | DTC P0102 00, P0103 00, P0111 00, P0112 00, P0113 00, P0114 00, P0116 00, P0117 00, P0118 00, P0119 00, P0121 00, P0122 00, P0123 00, P0221 00, P0222 00, P0223 00, P0335 00, P0336 00, or P0501 00 are not set.

• | The ignition voltage is greater than 10 V.

• | The start-up ECT is between -10 to +75°C (+14 to +167°F).

• | The start-up IAT is warmer than -10°C (+14°F).

• | The engine run time at minimum load is less than 50%.

• | The engine run time at maximum load is less than 90%.

• | The engine idle time is less than 40%.

### Conditions for Setting the DTC

The ECM determines that the modeled engine coolant temperature has met the calibrated target of +92°C (+198°F) and the measured engine coolant temperature (ECT) sensor is less than +81°C (+178°F) for greater than 2 s.

### Action Taken When the DTC Sets

• | DTC P0128 00 is a type B DTC.

• | The cooling fan will be commanded ON.

• | The A/C compressor will be commanded OFF.

### Conditions for Clearing the DTC

DTC P0128 00 is a Type B DTC.

### Diagnostic Aids

The following conditions can also set the DTC:

• | DTC P0128 00 occurring with insufficient vehicle interior heating is an indication of improper thermostat operation.

• | Corrosion on the engine coolant temperature (ECT) sensor terminals or the engine coolant temperature (ECT) sensor harness connector results in a greater voltage on the engine coolant temperature (ECT) sensor signal circuit, which is interpreted by the ECM as a colder engine coolant temperature (ECT) sensor.

• | A slight to moderate resistance in the engine coolant temperature (ECT) sensor signal circuit or low reference circuit will affect this diagnostic. This condition results in a greater voltage on the engine coolant temperature (ECT) sensor signal circuit, which is interpreted by the ECM as a colder engine coolant temperature (ECT) sensor.

### Reference Information

#### Schematic Reference

Engine Controls Schematics

#### Connector End View Reference

Component Connector End Views

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

#### Scan Tool Reference

Control Module References for diagnostic tester information

### Circuit/System Verification

1. Turn OFF the ignition.
2. Inspect the cooling system surge tank for the proper engine coolant level. Refer to Cooling System Leak Testing and Cooling System Draining and Filling.
3. If the ignition has been OFF for 8 h or greater, the intake air temperature, the ambient temperature, and the engine coolant temperature should be within +9°C (+16°F) of each other. Ignition ON, observe the scan tool IAT Sensor, Ambient Air Temperature, ECT Sensor parameters. Compare those sensor parameters to each other and also to the ambient temperature to determine if the ECT Sensor is skewed colder.
4. Engine running, observe the ECT Sensor parameter. The reading should be between -39 to +120°C (-38 to +248°F) depending on the current ambient temperature and the vehicle operating conditions.
5. Use the scan tool to verify the proper operation of the engine cooling system fan.

Note: A critical analysis of the operation of the thermostat is necessary to properly diagnose this DTC.

6. Verify the proper heat range and the operation of the thermostat. Refer to Thermostat Diagnosis.
7. Operate the vehicle within the conditions for running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the freeze frame/failure records data.

### Circuit/System Testing

1. Inspect the B74 Manifold Absolute Pressure (MAP) Sensor and the B66 Intake Air Temperature (IAT) Sensor elements for contamination and debris.
2. Ignition OFF, disconnect the harness connector at the B34 Engine Coolant Temperature (ECT) Sensor.
3. Inspect the B34 Engine Coolant Temperature (ECT) Sensor terminals and the harness connector for corrosion.
4. Ignition OFF for 90 s, test for less than 5 Ω between the low reference circuit terminal 2 and ground.

⇒ | If the resistance measures greater than 5 Ω, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the K20 Engine Control Module (ECM).

5. Disconnect the K20 Engine Control Module (ECM).
6. Test for less than 5 Ω between B34 Engine Coolant Temperature (ECT) Sensor terminal 1 and K20 Engine Control Module (ECM) terminal 35 (X1) for less than 5 Ω.

⇒ | If the resistance measures greater than 5 Ω, test the signal circuit for a high resistance.

7. If the circuits test normal, test or replace the B34 Engine Coolant Temperature (ECT).

### Component Testing

#### Static Test

Measure and record the resistance of the K20 Engine Control Module (ECM) Sensor at various ambient temperatures, then compare those measurements to the Temperature Versus Resistance.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Engine Coolant Temperature Sensor Replacement

• | Engine Coolant Thermostat Replacement

• | Control Module References for Engine Control Module replacement, setup, and programming