P0118: Engine Coolant Temperature Sensor Circuit High
TL;DR
P0118 = coolant temp sensor circuit high (reads too cold). Severity: medium. Drivable but can run rich and risk overheating if fans don’t engage. Top causes: failed ECT sensor (60%), wiring/connector open (30%), low coolant (10%). Fix: $20–$120 DIY.
Can I keep driving with P0118?
IF the temperature gauge reads normal and fans work → drive short-term to get it fixed; expect a rich mixture and worse economy. IF the gauge is erratic or the fans don’t run and the engine heats up → stop to avoid overheating damage.
Symptoms
- Check engine light on
- Hard starting (rich mixture)
- Worse fuel economy / black smoke
- Temperature gauge reading low/erratic
- Cooling fans may not engage (some vehicles)
Top causes (ranked by probability)
| Likely cause | Probability | Notes |
|---|---|---|
| Failed ECT sensor | 60% | Most common |
| Open wiring or connector fault | 30% | High signal = often an open circuit |
| Low coolant level / poor ground | 10% |
What does P0118 mean?
Technical explanation
A high ECT circuit voltage corresponds to a very low (often implausible) temperature reading, so P0118 sets when the signal is at/above the high threshold — typically an open circuit or failed sensor. The ECM may richen fueling (thinking the engine is cold) and, on some vehicles, fan logic tied to ECT can be affected, creating an overheating risk.
In simple terms
The coolant temperature sensor tells the computer how hot the engine is. P0118 means it’s reading “way too cold” (often because it’s broken or the wire is cut). The engine may run rich and, on some cars, the cooling fans might not turn on — so don’t ignore it.
How to diagnose P0118 (step by step)
- Read live coolant temp. A reading stuck very low (e.g., -40°C) points to an open circuit or failed sensor.
- Inspect the connector and wiring. Check for an open/corroded connector or damaged wire (high signal often = open).
- Check the sensor ground. A bad ground can mimic a high signal.
- Test the ECT sensor. Measure resistance vs temperature against spec.
- Replace the sensor. If wiring is good and the sensor is out of spec, replace it and top up coolant.
Repair options & cost
- Replace the ECT sensor
- Repair the open wiring/connector
- Fix the sensor ground
- Top up coolant if low
| DIY cost | $20–$120 |
|---|---|
| Workshop cost | $80–$250 |
| Repair time | 20–45 min |
Costs are local ballpark ranges and vary by region and vehicle.
Tools you’ll need
- OBD-II scanner (BlueDriver / ANCEL)
- Socket/wrench set
- Multimeter
Vehicle-specific notes
- P0118 (circuit high) usually means an open circuit or dead sensor; P0117 (circuit low) is the opposite.
- Some vehicles tie fan operation to ECT — watch for overheating.
- Bleed air after any coolant top-up.
Common mistakes to avoid
- Replacing the sensor without checking for an open connector/wire
- Ignoring overheating risk if fans depend on ECT
- Overlooking a bad ground
- Not topping up/bleeding coolant
Frequently asked questions
What does P0118 mean?
The coolant temperature sensor circuit is reading high voltage, which the ECM sees as an implausibly cold temperature — usually a failed sensor or an open wire.
Is it safe to drive with P0118?
Short-term, but watch the temperature gauge. On some vehicles the cooling fans depend on this sensor, so there is an overheating risk if it fails.
What is the difference between P0117 and P0118?
P0118 is circuit high (reads too cold, often an open circuit); P0117 is circuit low (reads too hot, often a short).
P0118 summary
| Meaning | Coolant temp sensor circuit high (reads cold) |
|---|---|
| Severity | Medium |
| Safe to drive? | Yes, short-term (watch temperature) |
| Top cause | Failed ECT sensor (60%) |
| DIY cost | $20–$120 |
| Shop cost | $80–$250 |