P257C sets when the control line of the vacuum reservoir valve reads too high, typically from a short to battery positive or a fault in the driver stage, so the valve can no longer be switched as commanded. The reservoir serves pneumatic actuators on diesel engines; GM's bulletin PI1260 on the 2014 Cruze diesel names leaks anywhere in the vacuum system that feeds the turbo vane and EGR actuators as a cause of P003A, P2598 and P2599, which shows how a fault here spreads to boost control. No manufacturer documents P257C itself. CARB 13 CCR 1968.2 requires circuit checks on such output components. P257A covers an open line and P257B a short to ground, while sensor faults on the reservoir itself are reported as P2525-P2529. The driver will mostly notice the warning lamp and, at worst, weaker boost.
Medium severity. Can I keep driving?If the car is running normally, drive in moderation to a repair shop. If it loses power or another warning appears, do not continue.
Symptoms
- Engine warning lamp on
- Weaker boost or EGR response in some cases
Most likely causes
- Control wire shorted to a battery-voltage line
- Internal fault in the reservoir control valve
- Faulty output stage in the engine control module
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
- It affects emissions.
What each character of P257C means
- P: Powertrain: engine and transmission
- 2: Generic code: same on every make
- 5: Speed and idle
- 7C: Specific fault in that system
Frequently asked questions
Can I keep driving with code P257C?
If the car is running normally, drive in moderation to a repair shop. If it loses power or another warning appears, do not continue.
What causes code P257C?
The most common causes are: control wire shorted to a battery-voltage line; internal fault in the reservoir control valve; faulty output stage in the engine control module.
Related codes
- P257AVacuum Reservoir Control Circuit/Open
- P257BVacuum Reservoir Control Circuit Low
- P2528Vacuum Reservoir Pressure Sensor Circuit High
Sources
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.