P0118 - Engine Coolant Temperature Circuit High Input
P0118 means the coolant temperature signal is reading colder than expected because the circuit is open or the signal is stuck high. It is not automatically just a bad coolant temperature sensor; an unplugged sensor, damaged wiring, connector corrosion, open circuit, failed sensor, or less common PCM and reference-voltage issue can set the same code.
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Common Next Steps
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Check coil boots, carbon tracking, and whether the miss follows a swap.
Inspect gap, fouling, wear, oil, coolant, and plug-well condition.
Move to injector balance, pulse, or leak-down checks if the misfire stays.
Use compression or leak-down testing when spark and fuel checks do not move the fault.
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Code Overview
P0118 means the coolant temperature signal is reading colder than expected because the circuit is open or the signal is stuck high. It is not automatically just a bad coolant temperature sensor; an unplugged sensor, damaged wiring, connector corrosion, open circuit, failed sensor, or less common PCM and reference-voltage issue can set the same code.
Common Causes
- IAT or ECT sensor unplugged, biased, shorted, open, or contaminated
- Connector corrosion, coolant intrusion, loose terminals, or damaged harness
- Low coolant, trapped air, thermostat fault, cooling fan fault, or real overheating for ECT-related codes
- Intake heat soak, damaged MAF/IAT assembly, or intake duct issue for IAT-related codes
- Reference voltage, ground, or PCM input issue after sensor and wiring checks
Symptoms
- Hard start
- Poor fuel economy
- Cooling fan runs unexpectedly
- Temperature gauge abnormal
- Rich or lean drivability symptoms
Diagnostic Steps
- Compare sensor readings to ambient temperature before start-up and to actual engine condition when warm
- Inspect connector fit, terminal tension, corrosion, coolant intrusion, rub-through, and harness routing
- Check reference voltage, ground, and signal response when the reading is stuck high or low
- For ECT faults, verify coolant level, trapped air, thermostat operation, fan operation, and actual temperature
- For IAT faults, inspect the intake path, MAF/IAT assembly, and heat-soak conditions
- Verify repair with plausible cold-start readings, warm-up behavior, and no pending sensor codes
Diagnostic Insight
P0118 should be treated as a coolant-temperature circuit-high fault before the sensor itself is condemned.
- A cold-looking ECT reading on a warm engine often points to an open circuit or poor connection.
- Connector corrosion, coolant intrusion, or broken wiring can mimic a failed sensor.
- Cold-engine scan data should be compared to ambient temperature before parts are replaced.
Repair Difficulty
Moderate
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