Parts
$15–40
P0117 — Engine Coolant Temperature Sensor Circuit Low
CAUTIONIs it safe to drive? Safe for a short drive, but with a bad coolant-temp signal the ECU mis-fuels (often lean) and the fan/timing strategy can act up — get it checked before any long drive.
What P0117 means
The coolant-temp signal is below the minimum credible voltage (often <0.1–0.2V), which the ECU scales as an impossibly HIGH coolant temperature or a wire shorted to ground. The ECU may cut cold-start enrichment and lean the mixture, and cooling-fan/timing strategy can misbehave.
Most likely causes (in order)
- ECT sensor signal wire shorted to ground
- Failed ECT sensor (internal short — cheap to swap, $15–40)
- Damaged wiring harness
- Water intrusion into the connector
Symptoms you might notice
- Engine runs rich when warm — black smoke possible
- Poor fuel economy
- Spark plugs foul quickly
- Heater may read unusually hot on gauge
- Check engine light on
What to check first
Disconnect the coolant-temp sensor with the key on — the reading should swing to the open-circuit (cold / over-range) value and P0118 may set. If the low reading persists with the sensor unplugged, the signal wire is shorted to ground in the harness; if it clears, the sensor is shorted internally.
Repair cost & difficulty
Labor
0.2–0.5 hr
Difficulty
Easy DIY