P0117: Engine Coolant Temperature Sensor Circuit Low
TL;DR
P0117 = coolant temp sensor circuit low (reads too hot). Severity: medium. Verify real temperature first — could be actual overheating. Top causes: failed ECT sensor (55%), wiring short to ground (30%), real overheating (15%). Fix: $20–$120 DIY.
Can I keep driving with P0117?
IF the engine temperature is genuinely normal → drive short-term to fix the sensor/wiring; fans may run constantly. IF the engine is actually hot/overheating → stop immediately to avoid serious engine damage.
Symptoms
- Check engine light on
- Cooling fans running constantly
- Temperature gauge reading high/erratic
- Possible leaner running
- Sometimes reduced power
Top causes (ranked by probability)
| Likely cause | Probability | Notes |
|---|---|---|
| Failed ECT sensor | 55% | Most common |
| Wiring short to ground / connector fault | 30% | Low signal = often a short |
| Genuine overheating | 15% | Rule this out first for safety |
What does P0117 mean?
Technical explanation
A low ECT circuit voltage corresponds to a very high (often implausible) temperature, so P0117 sets when the signal is at/below the low threshold — typically a short to ground or a failed sensor. The ECM may lean the mixture and run fans continuously. Always confirm whether the engine is actually overheating before assuming a sensor fault.
In simple terms
The coolant temperature sensor is reading "way too hot" (low voltage), usually because it’s shorted or broken. But first make sure the engine isn’t actually overheating — check the real temperature before blaming the sensor.
How to diagnose P0117 (step by step)
- Confirm actual temperature. Use an infrared thermometer or second gauge to check the engine isn’t truly overheating.
- Read live coolant temp. A reading stuck very high (e.g., 130°C+) with a cool engine points to a short or bad sensor.
- Inspect wiring/connector. Look for a signal wire shorted to ground or a damaged connector.
- 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.
Repair options & cost
- Replace the ECT sensor
- Repair the wiring short/connector
- Address genuine overheating if present
- Top up/bleed coolant if needed
| 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)
- Infrared thermometer
- Multimeter
Vehicle-specific notes
- P0117 (circuit low) usually means a short or dead sensor; P0118 (circuit high) is the opposite.
- Always rule out real overheating first — it’s the dangerous possibility.
- Constant-running fans are a classic P0117 symptom.
Common mistakes to avoid
- Assuming it’s only the sensor without checking real temperature
- Missing a wiring short to ground
- Ignoring constant fan operation
- Not bleeding coolant after service
Frequently asked questions
Is P0117 dangerous?
It can be if the engine is genuinely overheating. First confirm the real temperature; if the engine is hot, stop. If it’s a sensor/wiring fault with normal temperature, it’s a lower-urgency repair.
What is the difference between P0117 and P0118?
P0117 is circuit low (reads too hot, often a short); P0118 is circuit high (reads too cold, often an open circuit).
Why are my fans always on with P0117?
The ECM thinks the engine is very hot due to the low sensor signal, so it commands the cooling fans to run continuously.
P0117 summary
| Meaning | Coolant temp sensor circuit low (reads hot) |
|---|---|
| Severity | Medium |
| Safe to drive? | Yes if not truly overheating |
| Top cause | Failed ECT sensor (55%) |
| DIY cost | $20–$120 |
| Shop cost | $80–$250 |