Parts
$15–40
P0118 — Engine Coolant Temperature Sensor Circuit High
CAUTIONIs it safe to drive? The ECU thinks the engine is always stone cold and will over-enrich — safe short-term but will damage spark plugs and catalytic converter if left alone.
What P0118 means
The ECT signal wire is reading above the maximum credible voltage (above ~4.9V), corresponding to an impossibly low coolant temperature like -40°F. The ECU interprets this as an open circuit.
Most likely causes (in order)
- Loose, corroded, or unplugged ECT sensor connector (check first)
- Open circuit in the signal or ground wire
- Failed ECT sensor (internal open — sensor resistance went infinite)
- Wiring harness damage — broken wire near connector
Symptoms you might notice
- Check engine light on
- Engine runs rich — fuel smell possible
- Poor fuel economy
- Cooling fan may run full-time (ECU assumes worst case)
- Temperature gauge may read low or not move
What to check first
Measure resistance across the ECT sensor terminals directly (disconnected). At room temperature (~70°F) most ECT sensors measure 2,000–3,000 ohms; at operating temp (~195°F) expect 200–300 ohms. An infinite or open reading with the sensor in hand confirms a dead sensor.
Repair cost & difficulty
Labor
0.2–0.5 hr
Difficulty
Easy DIY