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.

### DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0118 Engine Coolant Temperature (ECT) Sensor Circuit High Voltage

### Conditions for Running the DTC

• | The engine is running for more than 1 minute.

| OR

• | The ignition is ON, and the intake air temperature (IAT) is more than -7°C (19°F).

| OR

• | The ignition is OFF, but the PCM has not powered down.

• | This DTC runs continuously within the enabling conditions in a 1-second loop.

### Conditions for Setting the DTC

The PCM detects that the ECT sensor parameter is less than -39°C (-38°F) for more than 5 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.

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
Schematic Reference: Engine Controls Schematics Connector End View Reference: Powertrain 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 | Important: The cooling fans will be commanded ON when certain engine coolant temperature (ECT) DTCs are set. Observe the ECT sensor parameter with a scan tool. Is the ECT sensor parameter less than the specified value? | -39°C (-38°F) | Go to Step 4 | Go to Step 3
3 |

1. Observe the Freeze Frame/Failure Records for this DTC.
2. Turn OFF the ignition for 90 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 4

Go to Testing for Intermittent Conditions and Poor Connections

4 |

1. Disconnect the ECT sensor.
2. Measure the voltage from the signal circuit of the ECT sensor to a good ground with a DMM. Refer to Circuit Testing and Wiring Repairs.

Is the voltage more than the specified value?

5.2 V

Go to Step 5

Go to Step 6

5 | Important: If a short to voltage occurs, the ECT sensor may be damaged. Test the ECT signal circuit for a short to voltage. Refer to Circuit Testing and Wiring Repairs. Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 12
6 |

1. Connect a 3-amp fused jumper between the signal circuit of the ECT sensor and the low reference circuit. Refer to Using Fused Jumper Wires.
2. Observe the ECT sensor parameter with a scan tool.

Is the ECT sensor parameter more than the specified value?

149°C (300°F)

Go to Step 10

Go to Step 7

7 |

1. Connect a 3-amp fused jumper wire between the signal circuit of the ECT sensor and a good ground. Refer to Using Fused Jumper Wires.
2. Observe the ECT sensor parameter with a scan tool.

Is the ECT sensor parameter more than the specified value?

149°C (300°F)

Go to Step 9

Go to Step 8

8 | Test the signal circuit of the ECT sensor for a high resistance or an open. Refer to Circuit Testing and Wiring Repairs. Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 12
9 | Test the ECT sensor low reference circuit for a high resistance or an open. 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 ECT signal circuit for a short to any 5-volt reference circuit. Refer to Circuit Testing and Wiring Repairs. Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 11
11 | Test for an intermittent and for a poor connection 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 an intermittent and for a poor connection at the powertrain control module (PCM). 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 PCM. 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 90 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