The engine is taking too long to reach normal operating temperature, or isn't reaching it at all. The computer expects the coolant to hit a certain temperature within a certain time after startup — if it doesn't, this usually means the thermostat is stuck open, letting coolant circulate through the radiator too early.
Symptoms
- Cabin heater blows cooler air than usual, especially on short trips
- Temperature gauge sits lower than normal or takes much longer to come up
- Slightly worse fuel economy (a cold engine runs a richer, less efficient mixture)
- No performance issues in most cases — the engine still runs fine, just cooler than intended
Likely causes
- Thermostat stuck partially or fully open — by far the most common cause
- A faulty coolant temperature sensor reporting an inaccurate (too low) reading
- Low coolant level, reducing the system's ability to hold heat efficiently
- Very cold ambient temperatures combined with a lot of short-trip driving, which can occasionally trigger this even with a healthy thermostat
How to diagnose it
- Watch the coolant temperature gauge or scan-tool live data during a drive — a thermostat stuck open shows a very slow, erratic climb to operating temperature
- Feel the upper radiator hose after startup — if it gets warm too quickly, coolant is flowing before the thermostat should be open
- Check coolant level and condition, and test the coolant temperature sensor's resistance against spec
Typical fixes & cost
- Replace the thermostat80–250 EUR
- Replace the coolant temperature sensor, if that's confirmed as the actual fault40–150 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