Powertrain, generic (extended)

P2B85 Engine Coolant Pump Underspeed

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

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.

Likely causes

Failed control board inside the electric water pump
Loose or poorly latched plug at the pump
Bearing wear or deposits slowing the impeller
Low supply voltage or high resistance in power or ground

Symptoms

  • 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

Repair estimate

DifficultyHardWorkshop job
Parts and labour€100–1,200
Time0.5–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

engine coolant pumpecm

Related codes

P2BA2P26CEP26CBP26CAP26D3P0217

References: gsi.ext.gm.com, gsi.ext.gm.com, static.nhtsa.gov, static.nhtsa.gov.