GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The engine coolant temperature (ECT) sensor is a variable resistor that measures the temperature of the engine coolant. The engine control module (ECM) supplies 5 volts to the ECT signal circuit and supplies a ground to the low reference circuit. If the ECM detects an excessively low ECT signal voltage, which is a high temperature indication, this DTC sets.

The following table illustrates the difference between temperature, resistance, and voltage:

ECT

ECT Resistance

ECT Signal Voltage

Cold

High

High

Warm

Low

Low

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0117 Engine Coolant Temperature (ECT) Sensor Circuit Low Voltage

Conditions for Running the DTC

The engine is running for more than 120 seconds.

Conditions for Setting the DTC

The ECM detects that the ECT sensor is more than 149°C (300°F) for more than 4 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 4 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

    • An overheating condition may cause this DTC to set.
    • After starting the engine, the ECT sensor temperature should rise steadily, then stabilize after the thermostat opens.
    • Test the ECT sensor at various temperature levels in order to evaluate the possibility of a skewed sensor. A skewed sensor can result in a driveability condition.
    • If the vehicle has set overnight, the ECT sensor and the intake air temperature (IAT) sensor values should display within 3°C (5°F). Refer to Temperature Versus Resistance .
    • For an intermittent condition, refer to Intermittent Conditions .

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine Controls Connector End Views

1

Did you perform the Diagnostic System Check - Engine Controls?

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Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Important: The cooling fans are commanded ON when certain DTCs are set.

  1. Turn ON the ignition with the engine OFF.
  2. Observe the Engine Coolant Temperature (ECT) sensor parameter with a scan tool.

Is the temperature more than the specified value?

149°C (300°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.

Did the DTC fail this ignition?

--

Go to Step 4

Go to Diagnostic Aids

4

  1. Disconnect the ECT sensor.
  2. Observe the ECT sensor parameter with a scan tool.

Is the temperature less than the specified value?

-39°C (-38°F)

Go to Step 6

Go to Step 5

5

Test the signal circuit of the ECT sensor for a short to ground and for a short to a low reference circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 10

Go to Step 8

6

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 10

Go to Step 8

7

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 10

Go to Step 9

8

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

Did you complete the replacement?

--

Go to Step 10

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9

Replace the ECM. Refer to Engine Control Module Replacement .

Did you complete the replacement?

--

Go to Step 10

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10

  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 11

11

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