P277AElectric/Auxiliary Transmission Fluid Pump B Hydraulic Leakage
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Can trigger limp mode
- Needs a drive cycle to confirm a fix
- How the car detects it
- 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.
- What you may notice
- Stop-start disabled or rough engine restarts
- Delayed drive engagement after standstill
- Warning lamp, possibly fluid traces under the gearbox
- Likely causes
- LikelyDamaged pump seal or O-ring at the pump mounting
- PossibleLow transmission fluid level letting the pump draw air
- PossibleCheck valve in the pump circuit not sealing
- RareWorn valve body bores causing internal leakage
- Repair
- Hard · Usually a job for a workshop.Typical cost: €100 – €1,800
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.