P257BVacuum Reservoir Control Circuit Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- How the car detects it
- P257B points to a vacuum reservoir control circuit whose voltage is too low, which in practice usually means a ground short on the line between the control unit and the reservoir valve, or a driver stage that cannot hold the line at its proper level. The valve manages a vacuum store used by pneumatic actuators such as EGR valves or turbocharger vanes, mostly on diesel engines. No carmaker publishes P257B with thresholds, so the logic follows the CARB 13 CCR 1968.2 rules for output components. With the line shorted, the valve may stay permanently switched, and the actuators behind it lose or keep vacuum regardless of demand; GM bulletin PI1260 shows that vacuum problems in such systems then show up as turbo position codes. P257A is the open circuit, P257C the high-side fault.
- What you may notice
- Engine warning lamp on
- Follow-up codes for EGR or turbocharger position
- Likely causes
- LikelyControl wire chafed through and shorted to ground
- PossibleShorted coil in the reservoir control valve
- RareDamaged output stage in the engine control module
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Often possible to do yourself.Typical cost: €30 – €300
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.