For 1990-2009 cars

### Circuit Description

The engine coolant temperature (ECT) sensor is a variable resistor, that measures the temperature of the engine coolant. The ECT sensor has a signal circuit and a low reference circuit. The powertrain control module (PCM) supplies 5 volts to the ECT signal circuit and a ground for the ECT low reference circuit. When the ECT is cold, the sensor resistance is high. When the ECT increases, the sensor resistance decreases. With high sensor resistance, the PCM detects a high voltage on the ECT signal circuit. With lower sensor resistance, the PCM detects a lower voltage on the ECT signal circuit. If the PCM detects an excessively high ECT signal voltage, which is a low temperature indication, DTC P0118 sets.

### Conditions for Running the DTC

The engine is running.

### Conditions for Setting the DTC

The ECT sensor temperature is less than -38°C (-36°F) for more than 15 seconds.

### 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.

### Diagnostic Aids

If a short to a separate 5-volt source occurs, this DTC may set. A continuity test to all other PCM circuits will be necessary in order to diagnose the specific circuit.

After starting the engine, the ECT should rise steadily to about 90° C (194° F) then stabilize when the thermostat opens.

Use the Temperature vs. Resistance table in order to test the ECT sensor at various temperature levels in order to evaluate the possibility of a skewed sensor. A skewed sensor could result in poor driveability concerns. Refer to Temperature Versus Resistance .

If a malfunction is not present at this time, refer to DTC P1115 .

If an intermittent condition is suspected, refer to Intermittent Conditions .

### Test Description

The number below refers to the step number on the diagnostic table.

1. This step tests for the proper operation of the circuit in the low voltage range. If the fuse in the jumper opens when you perform this test, the signal circuit is shorted to voltage.

#### DTC P0118

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
Schematic Reference: Engine Controls Schematics
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | Go to Diagnostic System Check - Engine Controls
2 |

1. Install a scan tool.
2. Turn ON the ignition, with the engine OFF.
3. Observe the ECT sensor temperature with a scan tool.

Does the scan tool indicate that the ECT sensor temperature is less than the specified value?

-38°C (-36°F)

Go to Step 4

Go to Step 3

3 |

1. Observe the Freeze Frame/Failure Records data 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 as specified in the supporting text or as close to the Freeze Frame/Failure Records data that you observed.

Does the DTC reset?

--

Go to Step 4

Go to Diagnostic Aids

4 |

1. Turn OFF the ignition.
2. Disconnect the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement .
3. Connect a 3 amp fused jumper wire between the signal circuit of the ECT sensor and the low reference circuit at the ECT sensor. Refer to Using Fused Jumper Wires in Wiring Systems.
4. Turn ON the ignition, with the engine OFF.
5. Observe the ECT sensor temperature with a scan tool.

Does the scan tool indicate ECT sensor temperature greater than the specified value?

138°C (280°F)

Go to Step 8

Go to Step 5

5 |

1. Turn OFF the ignition.
2. Connect a 3 amp fused jumper wire between the signal circuit of the ECT sensor and a known good ground.
3. Turn ON the ignition, with the engine OFF.
4. Observe the ECT sensor temperature with a scan tool.

Does the scan tool indicate ECT sensor temperature greater than the specified value?

138°C (280°F)

Go to Step 7

Go to Step 6

6 |

1. Test the signal circuit of the ECT sensor for:

• | A short to voltage

• | A high resistance

• | An open circuit

2. Repair the circuit as necessary. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 7

7 |

1. Test the ECT sensor reference low circuit for a high resistance or an open.
2. Repair the circuit as necessary. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 9

8 | Important: If a short to a separate voltage source occurs this DTC may set and damage the ECT sensor. If this is found to be the condition, a continuity test to all other PCM circuits will be necessary to diagnose the specific fault.

1. Inspect for poor connections at the harness connector of the ECT sensor.
2. Repair the connections as necessary. Refer to Circuit Testing and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 10

9 |

1. Inspect for poor connections at the harness connector of the PCM. Refer to Powertrain Control Module Replacement .
2. Repair the connections as necessary. Refer to Circuit Testing and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 11

10 | Replace the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement . Did you complete the replacement? | -- | Go to Step 12 | --
11 | Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | -- | Go to Step 12 | --
12 |

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 as specified in the supporting text.

Does the DTC run and pass?

--

Go to Step 13

Go to Step 2

13 | Observe the stored information, Capture Info., with a scan tool. Does the scan tool display any DTCs that you have not diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List | System OK