Engine Thermostat
What it is
The thermostat is a temperature-controlled valve between the engine and the radiator. It is a cheap mechanical part that has an outsized effect on how the engine runs.
What it does
It stays closed when the engine is cold so it warms up quickly, then opens to let coolant flow to the radiator and hold the right operating temperature. Stuck open, the engine runs too cold and sets P0128; stuck closed, it overheats.
Symptoms of failure
- P0128 "coolant temp below thermostat regulating temperature"
- Engine takes too long to warm up / heater stays cold
- Overheating (stuck closed)
- Poor fuel economy when running cold
- Temperature gauge reading abnormally low or high
Common fault codes
Which vehicles need it
Every liquid-cooled engine. A very common, inexpensive cause of P0128 and warm-up complaints.
Replacement cost
| DIY (part only) | $15–$60 |
|---|---|
| At a shop (parts + labor) | $150–$400 |
| Replacement interval | No set interval — replace on failure, ideally with the coolant change. |
| DIY difficulty | Moderate (DIY) — you must drain some coolant; access varies by engine |
| Recommended brands | Stant, Gates, Motorad, OEM |
Where to buy the part
We may earn a commission from purchases made through these links, at no extra cost to you.
Frequently asked questions
Can I drive with a stuck-open thermostat?
Short-term yes — the engine just runs cooler, hurting economy and heater output, and you will have a P0128 light. A stuck-closed thermostat is the dangerous one because it causes overheating; do not drive on that.
Is a P0128 always the thermostat?
Not always, but the thermostat is the most common cause. A faulty coolant temperature sensor or low coolant can also cause it, so verify before replacing.