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 P0117: Engine Coolant Temperature Sensor Circuit Low Voltage
DTC P0118: Engine Coolant Temperature Sensor Circuit High Voltage
DTC P1258: Engine Coolant Over Temperature - Protection Mode Active

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
ECT Sensor Signal | P0117 | P0118, P0119 | P0118, P0119¹ | P0116
Low Reference | -- | P0118, P0119 | P0118, P0119¹ | P0116
¹ Sensor or ECM 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: Engine operating in Closed Loop. Parameter Normal Range: -39°C to 142°C (-38° to 288°F) Varies with coolant temperature.
ECT Sensor Signal | 143°C (289°F) | -40°C (-40°F) | -40°C (-40°F)¹
Low Reference | -- | -40°C (-40°F) | -40°C (-40°F)¹
Internal ECM or ECT sensor damage may occur if the circuit is shorted to B+

### Circuit 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 volts to the ECT signal circuit and supplies a ground to the low reference circuit.

The following table illustrates the difference between temperature, resistance and voltage:

ECT | ECT Resistance | ECT Signal Voltage
--- | --- | ---
Cold | High | High
Warm | Low | Low

### Conditions for Running the DTC

#### P0117

• | The engine is operating.

• | The start up intake air temperature (IAT) is less than 72°C (161°F).

OR

• | The start up intake air temperature (IAT) is more than 72°C (161°F).

• | The engine run time is more than 59 seconds.

• | The DTC runs continuously once the above condition are met.

#### P0118

• | The ignition is ON or the engine is operating.

• | The DTC runs continuously once the above condition is met.

#### P1258

• | The ignition is ON or the engine is operating.

• | The DTC runs continuously once the above condition is met.

### Conditions for Setting the DTC

#### P0117

The ECM detects that the ECT is more than 140°C (284°F) for more than 4 seconds.

#### P0118

The ECM detects that the ECT is less than -42°C (-43.6°F) for more than 4 seconds. The scan tool is limited to -40°C (-40°F), so the diagnostic procedure uses -39°C (-38°F) to determine if there is an ECT condition.

#### P1258

The ECM detects that the ECT is greater than 1312°C (268°F) for longer than 2 seconds.

### Action Taken When the DTC Sets

DTC P0117, P0118 and P1258 are type B DTCs.

### Conditions for Clearing the MIL/DTC

DTC P0117, P0118 and P1258 are type B DTCs.

### Diagnostic Aids

• | Test the ECT sensor at various temperature levels in order to evaluate the possibility of a skewed sensor. A skewed sensor can result in a DTC or a driveability condition. Refer to Temperature Versus Resistance - Engine Coolant Temperature Sensor.

• | If the vehicle has sat overnight, the IAT sensor and the ECT sensor values should display within 3°C (5°F).

• | After starting a cold engine, the ECT sensor temperature should rise steadily, then stabilize after the thermostat opens.

• | High resistance in the circuits of either the ECT or IAT sensor may set a DTC.

### 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 scan tool information

### Circuit/System Verification

Engine idling for 1 minute. Observe the DTC information with a scan tool. DTC P0117, P0118 and P1258 should not set.

### Circuit/System Testing

1. Ignition OFF, disconnect the ECT sensor.
2. Ignition ON, verify that the ECT sensor parameter is at -40°C (-40°F).

⇒ | If greater than the specified range, test the signal circuit for a short to ground. If the circuit/connections test normal, replace the ECM.

Notice: Do NOT use a test lamp to test the continuity of the circuit. Damage to the control module may occur due to excessive current draw.

Important:

• | The ECT sensor shares its low reference circuit with other sensors. If the low reference circuit internal to the ECM is open, 1-5 volts will be measured on the low reference circuit due to a back-feed condition through the other sensors.

• | The ECM or the sensor may be damaged if the circuit is shorted to B+.

3. Test for less than 5 volts between the low reference circuit terminal and a known ground.

⇒ | If greater than the specified range, repair the low reference circuit for a short to B+ and replace the ECM.

4. Ignition OFF, remove the fuse that supplies B+ to the ECM.
5. Test for less than 5 ohms between the low reference circuit terminal and a known ground.

⇒ | If greater than the specified range, test the low reference circuit for an open/high resistance. If the circuit/connections test normal, replace the ECM.

6. Install the fuse that supplies B+ to the ECM.
7. Ignition ON, install a 3-A fusible link wire between the signal circuit terminal and the low reference circuit terminal, Verify the ECT sensor parameter is greater than 142°C (288°F).

⇒ | If less than specified range, test the signal circuit for a short to voltage or an open/high resistance. If the circuit/connections test normal, replace the ECM.

8. If all circuits/connections test normal, test or replace the ECT sensor.

### Component Testing

1. Ignition OFF, disconnect the harness connector of the ECT sensor.

Important: A thermometer can be used to test the sensor off the vehicle.

2. Test the ECT sensor by varying the sensor temperature while monitoring the sensor resistance. Compare the readings with the Temperature Versus Resistance - Engine Coolant Temperature Sensor. Verify that the resistance is within 5 percent of the specification.

⇒ | If the resistance vs temperature is not within 5 percent, replace the ECT sensor.

### Repair Instructions

Perform the Diagnostic Repair Verification.

• | Engine Coolant Temperature Sensor Replacement

• | Engine Control Module Replacement for engine control module replacement, setup and programming.