P2B85Engine Coolant Pump Underspeed
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- P2B85 applies to engines whose main water pump is electric: the pump turns clearly slower than commanded. On GM's 2.7-litre L3B (table 19OBDG03D) the filtered gap between commanded and reported speed must exceed a map value – 200 rpm at low commands, up to 800 rpm at 8,000 rpm – during 8 of 10 seconds, with 300–6,250 rpm requested and more than 11 V; there it is Type A, MIL on the first trip, while the 2025 table 25OBDG03B rates it Type B. GM program N202320510 replaced pumps with faulty circuit boards on 2020–2021 CT4, CT5, Silverado and Sierra with L3B or LSY: overheating, MIL, P2B85 and reduced power, in the end engine shutdown. Mercedes links P2B8500 on the M254 to loose plugs at the high-temperature pump. Pump running when off: P2BA2.
- What you may notice
- Rising coolant temperature and overheating warnings
- Reduced engine power to protect the engine
- Check engine light, often on the first trip
- Engine may be shut down if driving continues
- Likely causes
- LikelyFailed control board inside the electric water pump
- PossibleLoose or poorly latched plug at the pump
- PossibleBearing wear or deposits slowing the impeller
- RareLow supply voltage or high resistance in power or ground
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Hard · Usually a job for a workshop.Typical cost: €100 – €1,200
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.