Code
P1115
HUMMER
P — Powertrain
Engine Coolant Temperature (ECT) Sensor Circuit Intermittent High Voltage
AI status
Completed
Completed
100%
Causes
- Faulty ECT sensor (thermistor)
- Poor/loose/corroded connector or pins at the sensor
- Damaged wiring (chafing, breaks, exposed conductors) between sensor and PCM
- Short to 5V reference or other power source in the sensor circuit
- Poor engine/chassis ground affecting sensor return or PCM reference
- Intermittent PCM input fault or internal PCM damage
Symptoms
- Malfunction Indicator Lamp (MIL) illuminated
- Erratic or incorrect coolant temperature gauge or ECM displayed temperature
- Hard or rough cold starts, poor idle until engine warms
- Reduced fuel economy or drivability issues (rich/lean corrections)
- Transmission shift abnormalities if PCM uses coolant temp for shift logic
- Cooling fans may run incorrectly
What to check
- Read freeze frame and all stored codes; note operating conditions when code set
- Visually inspect ECT sensor connector and wiring for corrosion, damage, pin push-out, or repair patches
- Wiggle harness and connector while monitoring live ECT signal to try to reproduce the fault
- Measure ECT sensor signal voltage with key ON engine OFF and while warming engine; compare to expected behavior
- Measure sensor resistance (engine cold and hot) if accessible; compare to specification or expected NTC trend (resistance decreases as temperature rises)
- Check for short to battery 12V on the signal circuit and for short to ground
Signal parameters
- Sensor type: NTC thermistor (resistance decreases as temperature increases)
- Typical circuit: 5V reference from PCM, sensor to ground; sensor voltage varies between ~0.2–4.8 V depending on coolant temperature
- Cold engine: signal voltage typically near the upper end of the range (closer to reference); hot engine: voltage near lower end
- Intermittent high voltage condition: occasional spikes or sustained voltage near reference (open or short-to-power behavior)
- Resistance behavior: very high/open when circuit is open; decreasing measurable ohms as temperature increases
Diagnostic algorithm
- Retrieve codes and freeze frame data; confirm P1115 is current or historic and note engine temp when set.
- Visually inspect the ECT sensor connector and wiring for damage, corrosion, bent pins, or poor sealing. Repair as needed.
- Backprobe the ECT signal and measure voltage with key ON, engine OFF. Observe voltage while warming engine or while sensor is heated (hot water) to confirm voltage changes smoothly.
- Perform wiggle test on harness and connector while monitoring live data for intermittent jumps to high voltage.
- Disconnect the sensor and measure resistance across sensor terminals at ambient temperature; if possible, heat sensor and confirm resistance falls. Replace sensor if readings are out of expected NTC behavior or open.
- Check for short to 12V on the signal wire (with wiring disconnected) using a DMM; repair any short to power.
- Check continuity between the sensor connector and the PCM input pin; repair any opens or high-resistance connections.
- Verify PCM reference voltage and ground(s). If reference or ground is unstable, trace and repair ground or power issues.
- After repairs or component replacement, clear codes, perform functional test and a short drive cycle to confirm the fault does not return.
- If wiring and sensor are verified good and intermittent high persists, consider PCM input fault — consult manufacturer's service information before replacing PCM.
Likely causes
- Corroded or loose connector at the ECT sensor
- Broken/shorted wiring between ECT and PCM (intermittent open or short to power)
- Failed ECT sensor
Fault status
Status
Intermittent high voltage detected on the ECT sensor circuit. ECM flagged an implausible/high coolant temperature signal intermittently; diagnostic inspection of sensor, connector, and wiring recommended.
Repair difficulty: Medium
Diagnostic time: 0.5-2 hours
Repair manuals
Manual library for HUMMER
138
Browse 138 HUMMER manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.
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