$200–$550
DIY from $15
Moderate
If you do it yourself
Within a month
Before bigger damage
Common symptoms
- Check engine light on
- Temperature gauge reads low or takes a long time to warm up
- Heater blows lukewarm, especially in cold weather
- Slightly worse fuel economy
Common causes, ranked by likelihood
Based on typical diagnoses for P0128. Work from the most likely cause down before replacing expensive parts.
By far the most common cause.
How to diagnose it
- Confirm the coolant level is full and there's no air in the system.
- Watch coolant temperature with a scan tool — if it warms slowly or never reaches ~195°F, the thermostat is likely stuck open.
- Compare the temperature sensor reading against an infrared thermometer on the engine.
- Replace the thermostat if it's not regulating; it's the usual fix.
DIY vs shop cost
$200–$550
Parts + labour at an independent shop.
DIY can save up to $120
Make-specific notes
Stuck-open thermostats are extremely common on GM Ecotec engines and are the typical P0128 fix.
On many Chrysler/Dodge engines the thermostat is integrated into the housing, so the whole housing is replaced as a unit.
P0128 questions, answered
Can I drive with P0128?
Yes, it's safe. The engine just runs cooler than ideal, which can hurt fuel economy and heater performance. Replace the thermostat when convenient.
Will P0128 cause overheating?
No — it's the opposite. P0128 means the engine isn't getting warm enough, usually from a thermostat stuck in the open position.