P0117
Engine Coolant Temperature Sensor Circuit Low Input
The engine computer is getting a very low signal from the coolant temperature sensor. This usually means the sensor, wiring, or connector has a problem. It can make the engine run poorly and may cause hard starting, rich running, or fan problems.
- SEV
- 3/5
- DRIVE
- CAUTION
- DIY
- $20–$80
- SHOP
- $120–$350
Quick answer
AI-CITATION READYWhat it means
Can you drive with it?
Most common causes
- Failed engine coolant temperature sensor
- Coolant temperature sensor connector loose, corroded, or contaminated with coolant
- Damaged, shorted, or rubbed-through wiring in the ECT sensor circuit
Typical repair cost
DIY usually runs $20–$80. Typical shop repair lands around $120–$350, depending on the root cause.
01 / Definition
P0117 means the powertrain control module has detected a low input voltage from the engine coolant temperature (ECT) sensor circuit 1. On most vehicles, a low voltage signal indicates either a short to ground, a failed sensor, poor connector contact, or damaged wiring, and the PCM interprets this as an abnormally high coolant temperature. The fault can affect fuel mixture, idle quality, cooling fan operation, and cold-start enrichment.
02 / Drive status
With caution. You may be able to drive short distances, but the engine may run badly and fuel economy can drop. Do not ignore it, because incorrect coolant temperature readings can cause overheating or other damage if cooling fan control is affected.
03 / Symptoms
- Check engine light
- Poor fuel economy
- Hard starting, especially cold
- Rough idle or rich running
- Black smoke from exhaust
- Cooling fan running unexpectedly or not operating correctly
- Engine temperature gauge reading wrong on some vehicles
- Possible stalling or hesitation
04 / Causes
| 1 | Failed engine coolant temperature sensor | high |
|---|---|---|
| 2 | Coolant temperature sensor connector loose, corroded, or contaminated with coolant | high |
| 3 | Damaged, shorted, or rubbed-through wiring in the ECT sensor circuit | high |
| 4 | Low coolant level or air pocket around the sensor causing abnormal readings | medium |
| 5 | Corroded PCM connector or internal PCM fault | low |
05 / Diagnostic sequence
- 01Check coolant level and inspect for leaks before testing the sensor.
- 02Locate the engine coolant temperature sensor and inspect the connector for corrosion, coolant intrusion, bent pins, or loose fit.
- 03Inspect the harness near the sensor and along the engine for rubbing, broken insulation, or shorted wires.
- 04Use a scan tool to read live coolant temperature and compare it to a cold engine and ambient temperature.
- 05Measure sensor resistance and compare it to the manufacturer’s temperature/resistance chart.
- 06Check for reference voltage, ground, and signal integrity at the sensor connector.
- 07If wiring and sensor test good, check PCM connector condition and perform manufacturer-specific circuit testing.
06 / Repairs
| 1 | Top off coolant and repair any coolant leaks, then clear the code and recheck. | $0–$150 |
|---|---|---|
| 2 | Clean, repair, or replace the ECT sensor connector and damaged wiring. | $20–$150 |
| 3 | Replace the engine coolant temperature sensor if it fails resistance or live-data testing. | $20–$120 |
| 4 | Repair PCM-related wiring faults or perform PCM diagnosis if all other tests pass. | $100–$400 |
07 / Related codes
08 / FAQ
What does P0117 mean?
It means the engine coolant temperature sensor circuit is showing a signal that is too low, which usually points to a sensor or wiring fault.
Can a bad coolant temperature sensor cause hard starting?
Yes. If the PCM thinks the engine is already hot, it may not add enough fuel for a cold start.
Will P0117 make the cooling fan run all the time?
It can on some vehicles. The PCM may use a fail-safe strategy if it does not trust the coolant temperature reading.
Should I replace the sensor first?
Not always. Check coolant level, connector condition, and wiring first because those are common causes.
09 / Source and method
- DATA BASIS
- OBD-II REFERENCE + OBD2.HELP
- METHOD
- STATIC VALIDATION
- SAFETY
- INFORMATIONAL
This page combines OBD-II diagnostic reference data with OBD2.help generated diagnostic guidance for code meaning, likely causes, and repair direction.
Publishing uses deterministic schema and build validation, plus manual spot checks on representative pages before release.
Safety-critical diagnosis and repairs should be confirmed with a qualified mechanic, especially when the vehicle is misfiring, overheating, or losing power.