P2CEFAuxiliary Thermostat A Stuck Open
- 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
- P2CEF concerns a second thermostat that regulates its own part of the cooling system; stuck open means coolant passes through it before the intended temperature, so that circuit warms too slowly or stays too cool. Lotus documents the code in bulletin 131-26007 for the four-cylinder Emira, where thermostat A is the medium-temperature thermostat: the MIL may light and the driver may see a warning about low coolant temperature; the coolant is filtered for particles and the cold thermostat checked for a recessed piston and free travel in hot water. Mercedes-AMG lists P2CEF00 on M139 engines (CLA 45/GLA 45 facelift) with its own wording, an electrical fault of the thermostat, and blames debris, fixed by a revised thermostat. CARB requires monitoring of multi-thermostat systems.
- What you may notice
- Engine warning lamp
- Low coolant temperature warning or slow warm-up
- Weaker cabin heating in cold weather
- Likely causes
- LikelyDebris in the coolant holding the thermostat valve off its seat
- PossibleWorn or leaking wax element, piston no longer returns
- RareEarlier thermostat version superseded by the maker (Mercedes M139)
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €150 – €900
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.