OBD-II diagnostic trouble code
P0128Safe to drive
Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
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
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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.
Frequently asked questions
Can I drive with P0128?
In most cases, yes — P0128 is generally not an urgent safety issue and the car is usually safe to drive for a while. Still, get it checked soon, since the underlying cause can affect fuel economy or emissions the longer it's left.
How much does it cost to fix P0128?
Typical repair costs for {code} range from roughly 40 to 250 EUR, depending on the specific cause and your car — see the fixes listed above for a breakdown.
What causes P0128?
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
Reviewed by Artyom SemenovAutomotive Editor · Fact-checked by Yauheni Kapliarchuk, Editor-in-Chief