P0117 Code: Engine Coolant Temperature Sensor Low Input
Quick Answer
The engine coolant temperature sensor circuit voltage is lower than expected — the ECU thinks the engine is extremely hot.
At a Glance
What this code means
The ECT sensor's resistance changes with temperature — high resistance when cold, lower resistance when hot. P0117 means the sensor circuit voltage is below the minimum threshold, which typically indicates a short to ground or a failed sensor reading an unrealistically high temperature. The ECU may enrich the mixture or alter timing based on this incorrect reading.
Common causes
- Faulty ECT sensor (internal short)
- Short to ground in the sensor wiring
- Corroded or damaged sensor connector
- ECT sensor installed in the wrong coolant port
Symptoms you might notice
- Check engine light on
- Engine may run rich (ECU assumes engine is too hot, compensates incorrectly)
- Poor fuel economy
- Cooling fan may run continuously if the ECU acts on false temperature data
- Temperature gauge may read abnormally high or low
Can you still drive?
Yes, but monitor your actual temperature gauge carefully. If the engine is genuinely overheating, stop driving immediately. If the gauge is normal and only the CEL is on, it's likely just the sensor.
How serious is this code?
Moderate. A bad ECT sensor causes incorrect fuel trims and timing. More importantly, if the sensor is masking a real overheating event, it could lead to engine damage.
Before you replace parts
Parts replacement is often not the first step. Before buying anything, it's worth checking these basics:
- Verify actual coolant temperature with an infrared thermometer or by watching the temp gauge — a faulty sensor and a genuinely hot engine need very different repairs
- Inspect the coolant level and check for leaks before assuming the sensor is bad
How to troubleshoot it
- Check the coolant level — low coolant can cause temperature sensor reading issues
- Inspect the ECT sensor connector for corrosion or damage
- Use a multimeter to check sensor resistance at different temperatures and compare to spec
- Test for a short to ground in the wiring between the sensor and ECU
- Replace the ECT sensor if resistance is out of spec
Tools that may help
These are the types of tools commonly used when diagnosing this code. Having the right tool can save time and help confirm a diagnosis before spending money on parts.
- Multimeter
- OBD-II scanner with live coolant temp data
Estimated repair cost range
ECT sensor replacement: $40–$150 including the sensor. Labor adds $50–$150 depending on sensor location.
Costs vary significantly by vehicle, location, and whether you use OEM or aftermarket parts. These are general ranges for reference only.
When to call a mechanic
If you've completed the basic troubleshooting steps and the code keeps returning, or if you don't have access to the proper diagnostic tools, it's worth consulting a professional. A qualified technician can perform a full diagnosis and confirm the root cause before any parts are purchased. When in doubt, get a professional opinion — it can save you from an expensive misdiagnosis.
Real-world note
Verify the actual coolant temperature with an infrared thermometer or by watching the temperature gauge. If the real engine temperature is normal but the sensor reads extreme values, the sensor is faulty. If the engine is genuinely running hot, address the cooling system issue first.
Informational purposes only. This guide is for educational reference and is not a substitute for diagnosis by a qualified technician. Repair costs, causes, and symptoms may vary by vehicle make, model, year, and condition. Always consult a licensed mechanic before performing major repairs.