P257B
Vacuum Reservoir Control Circuit Low
The vacuum reservoir control circuit reads stuck low — short to ground or low-side driver stuck on. The actuator may be energised continuously when it should be off.
- Check engine light: Yes
- Emissions related: No
- Repair difficulty: medium
Common causes
Ordered by how often each turns out to be the cause.
- Control wire shorted to groundCommon
- Connector pins corroded - high contact resistanceLess likely
- Harness chafing through insulationLess likely
Parts involved
- Vacuum Reservoir Control Valve
P257B is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.