Quick answer: The P0118 code indicates that your car’s computer detects an abnormally high signal voltage from the Engine Coolant Temperature (ECT) sensor, usually caused by an open circuit or faulty sensor. This affects the engine management system, requiring prompt attention to avoid poor fuel economy and engine overheating risks.
If your Check Engine Light is on and displaying code P0118, your vehicle’s computer has lost an accurate reading of the engine’s coolant temperature. This article explains what triggers this trouble code, the symptoms you might notice while driving, and how to accurately diagnose and resolve the issue without unnecessary expenses.
What Does P0118 Mean?
Code P0118 stands for “Engine Coolant Temperature Sensor 1 Circuit High Input.” The ECT sensor operates like a thermistor, changing its internal resistance based on coolant temperature. Higher resistance produces a higher voltage signal sent to the Powertrain Control Module (PCM).
When the PCM detects a voltage signal above the maximum normal limit-typically exceeding 4.6 volts, which signals an extremely cold temperature-it assumes there is a broken wire, disconnected plug, or internal sensor failure. Because the computer can no longer determine actual engine temperature, it logs code P0118 and activates a safety routine to protect the engine.
Symptoms
- Check Engine Light illuminated on the instrument cluster
- Radiator cooling fans running constantly at full speed as a safety measure
- Hard engine starting, particularly when the engine is already warm
- Poor fuel economy caused by the computer delivering extra fuel
- Rough engine idle or stalling shortly after starting
- Black exhaust smoke caused by a rich air-fuel mixture
Possible Causes
Replacing the temperature sensor immediately is a common mistake; wiring issues and loose plugs trigger this code just as frequently.
- Disconnected, loose, or corroded ECT sensor wiring connector
- Broken, pinched, or damaged wires between the sensor and the computer
- Faulty Engine Coolant Temperature sensor with internal open circuit
- Low coolant level causing air pockets around the sensor tip
- Faulty Powertrain Control Module or outdated software (rare)
Check These Yourself Before Visiting a Mechanic
- Locate the ECT sensor and verify that the electrical connector is firmly snapped into place without corrosion on the pins.
- Check your coolant reservoir when the engine is cold to make sure coolant covers the minimum fill line.
- Inspect the visible wire harness coming from the sensor for fraying, rodent damage, or contact with hot exhaust parts.
How to Fix It
- Check and top off coolant levels, then bleed air from the cooling system if necessary.
- Clean corrosion from the sensor connector terminals using electrical contact cleaner.
- Measure the reference voltage (5 volts) and ground wire at the sensor plug using a digital multimeter.
- Test the sensor’s resistance with a multimeter and compare the reading against manufacturer specifications.
- Replace the Engine Coolant Temperature sensor if its internal resistance is out of range.
- Repair damaged wire harness sections or pin connections if voltage testing indicates a wiring fault.
Estimated Repair Costs (2026)
| Job | Parts ($) | Labor ($) |
|---|---|---|
| ECT Sensor Replacement | $20 – $60 | $70 – $130 |
| Wiring / Harness Repair | $10 – $30 | $90 – $180 |
| Diagnostic Fee (Basic) | $0 | $80 – $150 |
| Coolant Flush & Top-Off | $25 – $45 | $60 – $100 |
Note: these are 2026 estimates in US dollars; costs vary by make and model.
Frequently Asked Questions
Can I drive with the P0118 code?
You can drive short distances to a repair shop, but prolonged driving is not recommended. The computer defaults to a rich fuel mixture and runs the cooling fans constantly, which can lead to fouled spark plugs, poor fuel efficiency, and unmonitored engine overheating.
How do I clear the P0118 code?
You can clear the code using an OBD-II scanner after completing the necessary repair. If the underlying electrical circuit fault or sensor failure is not fixed, the code and Check Engine Light will return almost immediately upon starting the engine.
Does code P0118 mean my engine is overheating?
Not necessarily. Code P0118 points to an electrical circuit issue where the computer sees maximum voltage. However, because the computer cannot monitor real coolant temperatures while this code is active, actual overheating can occur without warning on your dashboard gauge.