Parts
$15–80
P0128 — Coolant Temp Below Thermostat Regulating Temp
SAFEIs it safe to drive? The engine isn't warming up fast enough. Safe to drive; you'll lose some economy and heater performance.
What P0128 means
The coolant didn't reach the expected temperature in the expected time — the engine runs colder than designed. Nearly always a thermostat stuck open.
Most likely causes (in order)
- Thermostat stuck open (the #1 cause)
- Low coolant level
- Faulty coolant temp sensor reading low
- Cold-weather over-cooling
Symptoms you might notice
- Check-engine light
- Weak heat from the heater
- Slightly worse mpg
What to check first
Watch coolant temp climb on a scan tool — if it never reaches ~195°F, replace the thermostat. Verify the sensor reads correctly first.
Repair cost & difficulty
Labor
0.5–2 hr
Difficulty
DIY-friendly