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 P2181: Engine Cooling System Performance

### Circuit/System Description

The engine control module (ECM) controls the pulse width modulated (PWM) thermostat. The thermostat controls coolant flow and regulates the engine operating temperature. Ignition voltage is supplied directly to the thermostat. The ECM controls the thermostat by grounding the control circuit with a solid state device called a driver.

### Conditions for Running the DTCs

• | DTCs P00B3, P00B4, P0101, P0102, P0103, P0112, P0113, or P0114 are not set.

• | The engine run time is between 70 seconds and 22 minutes.

• | The ECT sensor at start-up is between -20°C to +75°C (19-140°F).

• | The IAT sensor is between -7°C to +75°C (-4°F to +167°F).

• | The airflow into the engine is between 11-100 g/s.

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

### Conditions for Setting the DTC

The ECM detects the engine coolant thermostat is stuck open for 160 seconds.

### Action Taken When the DTC Sets

DTCP2181 is a Type B DTC.

### Conditions for Clearing the DTC

DTCP2181 is a Type B DTC.

### Diagnostic Aids

• | Insufficient vehicle interior heating is an indication of improper thermostat operation.

• | The scan tool Desired ECT Sensor and the ECT Sensor parameters should be within 5°C (9°F) when the engine is at operating temperature.

• | A resistance condition in the ECT sensor circuits may cause this DTC. This condition results in a greater voltage on the ECT sensor signal circuit, which is interpreted by the ECM as a colder ECT.

• | The thermostat has a mechanical fail-safe in case of an electrical condition with the thermostat heater. The mechanical thermostat will open at approximately 104°C (220°F). The mechanical thermostat will cycle from approximately 104°C (220°F) to approximately 98°C (208°F).

### Reference Information

#### Schematic Reference

Engine Cooling 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 scan tool information

### Circuit/System Verification

1. Ignition ON, observe the DTC information with a scan tool. DTC P00B3, P00B4, P00B6, P0117, P0118, P0480, P0481. P0597, P0598, and P0599 should not be set.

⇒ | If a DTC is set, refer to Diagnostic Trouble Code (DTC) List - Vehicle for further diagnosis.

2. Verify the coolant in the radiator surge tank is at the correct level and there are no engine coolant leaks. Refer to Cooling System Draining and Filling and Loss of Coolant
3. Engine idling for 15 minutes with the A/C OFF. Verify the engine cooing fan is not ON at all times. Refer to Cooling Fan Always On.

Note: Depending on ambient temperature, it may take up to 4 minutes for the temperature to increase greater than the specified value.

4. Command the Engine Coolant Thermostat Heater to 0 percent with a scan tool. Increase the engine speed to 3,000 RPM. Observe the scan tool ECT sensor parameter. The temperature should increase to greater than 102°C (215°F).

⇒ | If less than the specified value, replace the engine coolant thermostat heater.

5. 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.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Engine Coolant Thermostat Replacement