Engine Coolant Temperature (ECT) Sensor Circuit Intermittent High Voltage

Category

Powertrain - Computer and Auxiliary Controls

Severity

Check Engine Light

Applies to

Silverado, Sierra, Tahoe, Impala, Malibu

Related

P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature) on OBD Codes List

Findings, check these first

• Failing ECT sensor with intermittent internal open
• Loose ECT connector terminals at the thermostat housing
• ECT signal wire broken inside insulation
• Air pocket around the ECT sensor after cooling work
• Corroded PCM connector on the ECT signal pin

Standard procedure

1. Graph ECT on a Tech2 or GDS2 and watch for sudden -40 degree dropouts
2. Wiggle-test the ECT harness from sensor to PCM while monitoring data
3. Check ECT connector terminal tension and reseat firmly
4. Verify coolant is full and properly bled after recent repairs
5. Replace the ECT sensor only on failed resistance tests, then clear and road test

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