GM/Detroit 6.5L ENGINE CONTROLS – DTC P0118 Engine Coolant Temperature (ECT) Sensor Circuit High Voltage

Circuit Description
The engine coolant temperature (ECT) sensor is a thermistor that controls signal voltage to the PCM. When the engine is cold, the sensor resistance is high, therefore the PCM will see high signal voltage. As the engine warms, sensor resistance becomes less and voltage drops. The voltage measured across the thermistor is interpreted as a temperature.

Conditions for Running the DTC
• The PCM performs this DTC diagnostic continuously.
• The engine operation time is more than 8 minutes.

Conditions for Setting the DTC
• The engine coolant temperature is less than or equal to -40°C (-40°F).
• All of the diagnostic set conditions met for 2 seconds.

Action Taken When the DTC Sets
• The PCM illuminates the malfunction indicator lamp (MIL) on the second consecutive drive trip that the diagnostic runs and fails.
• The PCM records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the Failure Records will store this information. If the diagnostic reports a failure on the second consecutive drive trip, the Freeze Frame records the operating conditions at the time of failure and updates the Failure Records.

Conditions for Clearing the MIL/DTC
• The PCM will turn the MIL off after three consecutive trips without a fault condition.
• A History DTC clears after forty consecutive warm-up cycles, if this or any other emission related diagnostic does not report any failures
• The use of a scan tool.

Diagnostic Aids
Check harness routing for a potential short to voltage. After engine is started, the ECT temperature should rise steady to about 85°C (185°F). A skewed sensor could result in poor driveability complaints. Refer to Temperature vs Resistance.

An intermittent may be caused by any of the following conditions:
• A poor connection
• Rubbed through wire insulation
• A broken wire inside the insulation

Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis in Wiring Systems. If a repair is necessary, then refer to Wiring Repairs or Connector Repairs in Wiring Systems.

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
2. This step determines if P0118 is a hard failure or an intermittent condition.
3. This step will determine if there is a wiring problem or a malfunctioning PCM.
9. This step determines if there is a short to voltage on the signal circuit. A short to voltage increases current flow through the sensor which overwhelms the sensor. This doesn’t allow the sensor to pull down the circuit to the correct voltage and thus display the correct temperature.
10. After repairing the short to voltage, check the sensor for proper operation.


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