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For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2005 🢒 TopKick C-Series (Conventional) C6/C7/C8 🢒 Engine 🢒 Engine Controls - 7.8L 🢒 DTC P0118

Circuit Description

The engine coolant temperature (ECT) sensor is a variable resistor, sometimes called a thermistor, that measures the temperature of the engine coolant. The engine control module (ECM) supplies 5 volts to the ECT signal circuit. When the ECT is cold, the sensor resistance is high. When the ECT increases, the sensor resistance lowers. With high sensor resistance, the ECM detects a high voltage on the ECT signal circuit. With lower sensor resistance, the ECM detects a lower voltage on the ECT signal circuit. If the ECM 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 ignition is ON.
    • The battery voltage is between 10-16 volts.

Conditions for Setting the DTC

The ECM detects that the ECT signal voltage is more than 4.95 volts.

Action Taken When the DTC Sets
    • The ECM illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
    • The ECM records the operating conditions at the time the diagnostic fails. The ECM stores this information in the Failure Records.
    • The ECM uses an ECT value of -29°C (-20.2°F) for engine starting.
    • The ECM uses an ECT value of 0°C (32°F) for engine running.
    • The ECM closes the exhaust gas recirculation (EGR) valve.
    • The ECM disables oil pressure sensor diagnostics.

Condition for Clearing the DTC
    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • Clear the MIL and the DTC with a scan tool.

Diagnostic Aids
    • After starting the engine, the ECT should rise steadily to about 80°C (176°F), then stabilize when the thermostat opens.
    • Use the Temperature vs. Resistance table to test the ECT sensor at various temperature levels to evaluate the possibility of a skewed sensor. A skewed sensor could result in poor driveability concerns. Refer to Temperature Versus Resistance .
    • If an intermittent condition is suspected, refer to Intermittent Conditions .

Test Description

The numbers below refer to the step numbers 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.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check - Vehicle?

--

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

  1. Observe the Failure Records 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. 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 3

Go to Diagnostic Aids

3
  1. Turn OFF the ignition.
  2. Disconnect the engine coolant temperature (ECT) sensor connector.
  3. Connect a DMM between the signal circuit and the low reference circuit of the ECT sensor harness.
  4. Turn ON the ignition, with the engine OFF.

Is the DMM voltage more than the specified range?

4.5-5.5 V

Go to Step 4

Go to Step 5

4

Important: The ECT sensor may be damaged if the circuit is shorted to a voltage source.

  1. Test the signal circuit for a short to voltage circuit.
  2. Repair the circuits, as necessary.

Did you find and correct the condition?

--

Go to Step 13

Go to Step 12

5
  1. Turn OFF the ignition.
  2. Disconnect the ECT sensor connector.
  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.
  4. Turn ON the ignition, with the engine OFF.
  5. Observe the ECT Sensor parameter with a scan tool.

Is the ECT Sensor parameter more than the specified value?

120°C (248°F)

Go to Step 9

Go to Step 6

6
  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 parameter with a scan tool.

Is the ECT Sensor parameter more than the specified value?

120°C (248°F)

Go to Step 8

Go to Step 7

7
  1. Test the signal circuit between the engine control module (ECM) and the ECT sensor for the following conditions:
    2. • An open circuit
    • High resistance
  2. Repair the circuits, as necessary.

Did you find and correct the condition?

--

Go to Step 13

Go to Step 10

8
  1. Test the low reference circuit between the ECM and the ECT sensor for the following conditions:
    2. • An open circuit
    • High resistance
  2. Repair the circuits, as necessary.

Did you find and correct the condition?

--

Go to Step 13

Go to Step 10

9
  1. Turn OFF the ignition.
  2. Disconnect the ECT sensor.
  3. Inspect for poor connections at the harness connector of the ECT sensor.
  4. Repair the circuits, as necessary.

Did you find and correct the condition.

--

Go to Step 13

Go to Step 11

10
  1. Turn OFF the ignition.
  2. Disconnect the ECM sensor.
  3. Inspect for poor connections at the harness connector of the ECM.
  4. Repair the circuits, as necessary.

Did you find and correct the condition.

--

Go to Step 13

Go to Step 12

11

Replace the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement .

Did you complete the replacement?

--

Go to Step 13

--

12

Important: The new ECM must be programmed.

Replace the ECM. Refer to Engine Control Module Programming and Setup , Engine Control Module Replacement/Restore Fuel Rate Data , and Fuel Injector Flow Rate Programming .

Did you complete the replacement?

--

Go to Step 13

--

13
  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. 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 14

14

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