Powertrain, generic (extended)

P277A Electric/Auxiliary Transmission Fluid Pump B Hydraulic Leakage

Check engine lightOn
EmissionsNot relevant
Drive cycleRequired
Limp modePossible

P277A is not an electrical fault. The motor of auxiliary pump B runs, yet the oil it delivers escapes before pressure can build, so the module concludes that the hydraulic path around the pump leaks. Typical suspects are a damaged pump seal or O-ring, a check valve that does not close, worn bores in the valve body or a fluid level so low that the pump draws air. No manufacturer documents P277A publicly. How such a test can look is shown by GM's 2025 TCM tables for pump A on a CVT: during an engine auto-start the pump is commanded, and if both pulley pressure sensors stay under 200 kPa while more than 400 kPa is requested, a performance code follows. P0B0C is the leakage code for pump A. Without pressure, stop-start restarts may be harsh or disabled.

Likely causes

Damaged pump seal or O-ring at the pump mounting
Low transmission fluid level letting the pump draw air
Check valve in the pump circuit not sealing
Worn valve body bores causing internal leakage

Symptoms

  • Stop-start disabled or rough engine restarts
  • Delayed drive engagement after standstill
  • Warning lamp, possibly fluid traces under the gearbox

Repair estimate

DifficultyHardWorkshop job
Parts and labour€100–1,800
Time1–6 h

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

Parts involved

aux trans pump bhydraulic circuitvalve body seals

Related codes

P0B0CP27A3P277CP276D

References: gsi.ext.gm.com, static.nhtsa.gov, law.cornell.edu.