GM Service Manual Online
For 1990-2009 cars only

Object Number: 891260  Size: MF

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 engine control module (ECM) 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 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.

Conditions for Running the DTC

The engine has been running for more than 250 seconds.

Conditions for Setting the DTC

The ECT sensor parameter is less than -39°C (-38°F) for more than 10 seconds.

Action Taken When the DTC Sets

    • The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/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.
    • 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 this DTC is determined to be intermittent, 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.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine Controls Connector End Views or Engine Control Module Connector End Views

1

Did you perform the Diagnostic System Check-Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Observe the DTC info parameter with a scan tool.

Is DTC P0641 set?

 

Go to DTC P0641

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.

Did the DTC fail this ignition?

--

Go to Step 4

Go to Diagnostic Aids

4

  1. Disconnect the engine coolant temperature (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 in Wiring Systems.

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 in Wiring Systems.

Did you find and correct the condition?

 

Go to Step 15

Go to Step 12

6

  1. Connect a 3-amp fused jumper wire between the signal circuit of the ECT sensor and the low reference circuit of the ECT sensor. Refer to Using Fused Jumper Wires in Wiring Systems.
  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.
  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 for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

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 for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 12

10

Important: If a short to voltage occurs this DTC may set and damage the ECT sensor.

Test the signal circuit of the ECT sensor for a short to any 5-volt reference circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

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 in Wiring Systems.

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 engine control module (ECM). 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 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 ECM. Refer to Engine Control Module Replacement .

Did you complete the replacement?

--

Go to Step 15

--

15

  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 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

System OK