Powertrain, generic (extended)

P257B Vacuum Reservoir Control Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modeNo

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.

Likely causes

Control wire chafed through and shorted to ground
Shorted coil in the reservoir control valve
Damaged output stage in the engine control module

Symptoms

  • Engine warning lamp on
  • Follow-up codes for EGR or turbocharger position

Repair estimate

DifficultyMediumDIY possible
Parts and labour€30–300
Time0.3–2 h

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

Parts involved

vacuum reservoir control valve

Related codes

P257AP257CP2526

References: law.cornell.edu, static.nhtsa.gov.