Code
P0118
HUMMER
P — Powertrain
Engine Coolant Temperature (ECT) Sensor Circuit High Voltage
AI status
Completed
Completed
100%
Causes
- Open or high-resistance wiring between ECT sensor and PCM
- Disconnected, corroded, or damaged ECT sensor connector
- Failed (open) ECT sensor or internal sensor wiring break
- Short to vehicle battery voltage or pull-up circuit fault
- Poor or missing sensor ground at sensor or ECU
- PCM/ECM internal input fault (less common)
Symptoms
- Coolant temperature gauge or dash readout shows abnormally low/‘cold’ temperature
- Extended open-loop fueling: rough cold idle, poor fuel economy, increased emissions
- Possible hard cold start or sluggish warm-up
- Radiator fan may not run when expected (PCM thinks engine is cold)
- Check Engine Light (MIL) illuminated, possible stored freeze-frame data
What to check
- Retrieve DTCs and freeze-frame data with a scan tool; note engine temp at set condition
- Visually inspect ECT sensor connector and wiring for damage, corrosion, disconnection
- Back-probe sensor connector with ignition ON (engine OFF) to measure circuit voltage
- Measure sensor resistance at the sensor with engine cold and compare to expected values
- Confirm PCM 5V reference (or system reference) and sensor ground are present and stable
- Wiggle wiring harness around engine and connector while monitoring live ECT voltage for intermittency
Signal parameters
- Typical reference supply to sensor circuit: ~5.0 V (verify factory spec)
- High-voltage fault threshold: often >4.5–4.8 V on ECT circuit (varies by vehicle) — consult service manual
- Typical ECT sensor resistance (NTC thermistor, approximate): ~2–4 kΩ at ~20°C (cold), ~200–500 Ω at ~80°C (hot) — compare to manufacturer chart
- Expected ECT voltage range with key ON/engine OFF: varies with ambient temp; should not be near full battery voltage if sensor connected and correct
Diagnostic algorithm
- Connect a scan tool, confirm P0118 and review freeze-frame data and live ECT value.
- Visually inspect the sensor, connector, and wiring for obvious damage, corrosion, or loose pins; repair as needed.
- With ignition ON (engine OFF), back-probe the ECT signal wire and measure voltage. If voltage is above the high threshold (typically >4.5 V), suspect open/high resistance or short to battery voltage.
- Verify reference voltage from PCM at the connector (usually ~5V) and check sensor ground continuity to chassis ground/PCM ground.
- Disconnect the sensor and measure resistance across sensor terminals with a multimeter at a known ambient temperature; compare to spec chart. An open or out-of-spec resistance indicates a bad sensor.
- If harness wiring or connector shows no problem but voltage still high, perform continuity checks from the sensor connector to the PCM pin for opens and for shorts to voltage and to ground.
- Repair wiring or connector faults; replace ECT sensor if resistance/behavior is incorrect.
- After repairs, clear codes, perform a cold start and warm-up while monitoring live ECT values to confirm correct operation and that code does not return.
- If wiring and sensor check good but problem persists, consider PCM input fault and follow manufacturer procedures for module testing/confirmation.
Likely causes
- Broken or chafed harness wire near engine or connector causing open circuit
- Corroded terminals or water intrusion in sensor connector
- ECT sensor failure (open or out-of-spec resistance)
- Poor ground at sensor or module causing incorrect voltage reading
- Aftermarket part or recent service left connector not fully seated
Fault status
Status
PCM reports ECT sensor circuit voltage higher than the allowed range (indicates open/high-resistance circuit or wiring/connector fault).
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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Repair manuals
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138
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