Code
P1115
OLDSMOBILE
P — Powertrain
Engine Coolant Temperature ECT Sensor Circuit Intermittent High Voltage IAT B Circuit High Input
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UK: 38
EN: 86
RU: 58
AI status
Completed
Completed
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Causes
- Faulty ECT or IAT B thermistor sensor (open or internal failure)
- Damaged, corroded or loose connector or pins at sensor
- Broken wiring, intermittent open, or short to battery voltage in the sensor circuit
- Short to 5V reference or voltage feed at the sensor signal wire
- Poor engine/ECM ground or high resistance in ground path
- Faulty or intermittent PCM/ECM input circuitry
Symptoms
- Malfunction Indicator Lamp (MIL/CEL) illuminated intermittently
- Erratic or incorrect engine temperature readings on scan tool or dash gauge
- Hard cold starts, poor idle, or drivability issues under some conditions
- Reduced fuel economy or incorrect fuel trim behavior
- Freeze frame or stored data showing intermittent high sensor voltage spikes
What to check
- Scan for codes and freeze-frame/fuel trim data; monitor live ECT and IAT B sensor values with a scan tool
- Perform a visual inspection of sensors, connectors and wiring for damage, corrosion or water intrusion
- Backprobe the sensor signal wire and measure voltage with key ON/engine OFF and while cranking/warm-up
- Measure sensor resistance out of circuit and compare with expected thermistor behavior (resistance decreases as temperature increases)
- Check continuity from sensor signal pin to ECM and for shorts to battery voltage or ground
- Wiggle harness and connectors while monitoring live data to reproduce intermittent behavior
Signal parameters
- Circuit uses an ECM reference (typical ~5V) and returns a signal voltage to ECM; expected signal range typically ~0.5–4.5 V depending on temperature
- Thermistor characteristic: resistance decreases as temperature increases (typical resistance range across operating temps ~200 ohm–10 kohm depending on sensor design)
- Intermittent high input means the ECM sees sudden spikes toward the reference voltage (near battery/reference voltage) when it expects lower voltages for given temperature
- With a disconnected/open thermistor the signal will often read high (near reference voltage); a short to ground results in very low voltage
Diagnostic algorithm
- Confirm code and record freeze-frame data. Clear codes and attempt to reproduce. Monitor ECT and IAT B live data while warming/cooling engine.
- Visually inspect sensor(s) and harness. Repair any obvious damage or corrosion at connectors and pins.
- Backprobe the signal and reference pins at the sensor connector. With key ON (engine OFF) verify reference voltage present and check signal voltage behavior while manipulating harness.
- Remove sensor and measure its resistance at known ambient temperature; compare to manufacturer spec or verify thermistor behavior by heating/cooling sensor with warm water and checking for smooth resistance change.
- Perform continuity check from sensor signal pin to ECM input pin and check for unintended continuity to battery power or ground. Repair any opens/shorts.
- If wiring and connectors good, swap with known-good sensor (if available) or replace sensor and retest. If problem persists, suspect ECM input circuit and consult service info/perform bench tests or replace ECM if confirmed.
- After repairs, clear codes and road test to verify no recurrence. Document intermittent tests if needed.
Likely causes
- Corroded/loose sensor connector allowing intermittent high-voltage reading
- Broken or chafed signal wire making intermittent contact to a 5V source
- Failed thermistor inside ECT or integrated IAT/ECT sensor (open circuit when cold)
- Short to battery voltage on the signal wire (pinched harness)
- Poor ground at engine harness causing unstable reference/reading
Fault status
Status
Intermittent high-voltage condition detected on the Engine Coolant Temperature (ECT) sensor circuit / Intake Air Temperature (IAT) B circuit. ECM flagged a high input or voltage spikes inconsistent with expected thermistor behavior. Inspect sensor, connector, wiring, grounds and ECM input for faults.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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