Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Engine Coolant Temperature Sensor
The Engine Coolant Temperature (ECT) sensor is a variable resistor that measures the temperature of the engine coolant. ECT sensor is located on side of left cylinder head. The PCM supplies 5 volts to the signal circuit and a ground for the ECT low reference circuit. When ECT is low, sensor resistance is high. When ECT is high, sensor resistance is low. The PCM uses this input for engine controls and enabling criteria for diagnostics. The internal clock of the PCM will record the amount of time ignition is off. At restart, PCM will compare the temperature difference between the ECT and the IAT sensor. Before failing this test, the PCM will perform a calculation to determine the presence of a block heater. If PCM detects that the temperature difference is not within the calibrated amount after the ignition off time, a DTC will set. To diagnose, see SELF-DIAGNOSTICS - 8.1L AVALANCHE, SIERRA, SILVERADO, SUBURBAN & YUKON XL article.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.