GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 Envoy - 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 more than 60 seconds.

Conditions for Setting the DTC

The ECT sensor temperature is less than 50°C (122°F) for more than 6.25 seconds.

Action Taken When the DTC Sets
    • The control module illuminates the malfunction indicator lamp (MIL) if a failure is detected during 2 consecutive key cycles.
    • The control module sets the DTC and records the operating conditions at the time the diagnostic failed. The failure information is stored in the scan tool Freeze Frame/Failure Records.

Conditions for Clearing the MIL or DTC
    • The control module turns OFF the MIL after 3 consecutive drive trips when the test has run and passed.
    • A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles. A warm-up cycle occurs when the coolant temperature has risen 22°C (40°F) from the startup coolant temperature and the engine coolant reaches a temperature that is more than 70°C (158°F) during the same ignition cycle.
    • Use a scan tool in order to clear the DTCs.

Diagnostic Aids
    • If a short to a separate 5-volt source occurs, this DTC may set. If this is the problem, a continuity test to all other PCM circuits will be necessary to diagnose the specific circuit.
    • After starting the engine, the ECT should rise steadily to about 90°C (194°F) then stabilize after 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 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.

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. Connect a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. With a scan tool, observe the ECT sensor temperature.

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. With a scan tool, observe the ECT sensor temperature.

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. With a scan tool, observe the ECT sensor temperature.

Does the scan tool indicate that the ECT sensor temperature is 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 the following condition:
    2. • 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 IAT sensor. If this is found to be the condition, a continuity test to all other PCM circuits willl be necessary to diagnose the specifc fault.

  1. Inspect for poor connections at the harness connector of the ECT sensor.
  2. Repair the connections as necessary. 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. 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 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. Use the scan tool in order to clear the DTCs.
  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

With a scan tool, observe the stored information, Capture Info.

Does the scan tool display any DTCs that you have not diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

System OK