GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 Safari - 4WD 🢒 DTC 🢒 DTC P0118

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 has been running for more than 60 seconds.

Conditions for Setting the DTC

The ECT sensor temperature is less than -38°C (-36°F) for 20 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. If this is found to be the problem a continuity test to all other PCM circuits will be necessary to diagnose the specific circuit.

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, a short to voltage in the signal circuit is indicated.

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 engine coolant temperature (ECT) sensor parameter with a scan tool.

Does the scan tool indicate that the ECT sensor parameter 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 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.

Does the DTC reset?

--

Go to Step 4

Go to Diagnostic Aids

4

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

  1. Turn OFF the ignition.
  2. Disconnect the ECT sensor.
  3. Connect a 3-amp fused jumper wire between the signal circuit of the ECT sensor and the low reference circuit.
  4. Turn ON the ignition, with the engine OFF.
  5. Observe the ECT sensor parameter with a scan tool.

Does the scan tool indicate ECT sensor parameter more 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 parameter with a scan tool.

Does the scan tool indicate ECT sensor parameter more than the specified value?

138°C (280°F)

Go to Step 7

Go to Step 6

6
  1. Test the signal circuit for the conditions listed below. Refer to Circuit Testing in Wiring Systems.
    2. • A short to voltage
    • High resistance
    • An open circuit
  2. If a condition is found, refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 7

7

Test the low reference circuit for high resistance or for an open. Refer to Circuit Testing Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 9

8

Important: An ECT signal circuit shorted to a separate 5-volt source will test the same as a bad ECT sensor.

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 in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 10

9
  1. Disconnect the powertrain control module (PCM).
  2. Test for an intermittent and for a poor connection at the PCM. Refer to Testing for Intermittent Conditions and Poor Connections 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. 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 13

13

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

System OK