P0128Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- After a cold start the thermostat stays closed so the engine warms up quickly. CARB requires the coolant to reach a temperature at most 11 °C below the thermostat's rated value within a set time. GM models the heat put in (fuel energy minus losses to ambient, cabin heater and coasting) and fails if enough energy has gone in without coolant reaching about 65 °C (55 °C in cold weather). Ford compares the measured value with a modelled one, typically failing below 78 °C with an 89 °C thermostat. The test runs once per drive cycle, pauses with much idling or below about −7 °C, and needs two trips. Usually the thermostat is stuck open; a sensor reading too low is the other suspect.
- What you may notice
- Check engine light on
- Temperature gauge stays low or drops while driving
- Heater does not get properly warm
- Higher fuel consumption
- Likely causes
- LikelyThermostat stuck open or opening too early
- PossibleCoolant sensor reading too low, or corrosion and resistance in its circuit
- PossibleLow coolant level or air in the system
- RareRadiator fan running constantly (relay stuck, wrong control)
- RareDriving pattern or conditions (much idling, extreme cold, strong heating)
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Often possible to do yourself.Typical cost: €60 – €400
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.