The coolant temperature sensor's signal to the computer is reading lower than physically possible — essentially, the computer thinks the engine is far colder than it really is, which usually means an electrical fault rather than an actual cooling problem.
Symptoms
- Cooling fan may run constantly, even with a cold engine
- Poor fuel economy — the computer keeps running a cold-start-style rich mixture
- Rough idle, especially after the engine has warmed up
- Temperature gauge (if it reads directly from this sensor) may sit unusually low or peg at maximum
Likely causes
- Short circuit to ground in the sensor's wiring — the most common cause of a "low input" fault specifically
- Failed coolant temperature sensor itself
- Corroded or damaged connector at the sensor
How to diagnose it
- Check the sensor connector for corrosion, moisture, or damage
- Test the sensor's resistance at different (known) temperatures against the spec table in the service manual
- Check the wiring harness for a short to ground between the sensor and the ECU if the sensor itself tests fine
Typical fixes & cost
- Replace the coolant temperature sensor40–150 EUR
- Repair a shorted or damaged wiring harness section80–250 EUR
Code names are compiled from open/standardized SAE and ISO references. Explanations, symptoms, causes and fixes are original. Covers generic (P0/C0/B0/U0) codes only — manufacturer-specific codes are planned for a future update.
AS
Reviewed by Artyom SemenovAutomotive Editor · Fact-checked by Yauheni Kapliarchuk, Editor-in-Chief