P257C
Vacuum Reservoir Control Circuit High
Vacuum supply / control
Generic OBD-II
Medium
What it means
P257C means the ECM sees a higher-than-expected voltage on the vacuum reservoir control circuit, usually from a short to voltage or an open. The vacuum reservoir stores engine vacuum for systems that need it, which on some vehicles includes the brake booster and various actuators. It's typically a wiring or control-valve fault; the engine runs normally, but it's worth confirming vacuum-assisted braking is unaffected.
Medium
Can I keep driving? Usually yes, but check brake-pedal feel and any vacuum-operated systems, since the reservoir supports engine vacuum.
Common symptoms
What you'll notice
- Check-engine light on
- Often no obvious symptom beyond the stored code
- Possible weaker vacuum-assisted brake feel on vehicles where the reservoir feeds the booster
- HVAC or other vacuum-actuated controls may behave inconsistently
Common causes
Most likely first
- Short to voltage or open in the vacuum reservoir control circuit (drives the signal high)Likely
- Faulty vacuum reservoir control valve/solenoidCommon
- Corroded or loose connector at the control valveCommon
- Damaged or chafed control-circuit wiringLess often
- Faulty ECM/PCM driver (least likely)Less often
How to diagnose & fix
Work through it in order
- Read freeze-frame data and confirm the control signal is reading high.
- Locate the vacuum reservoir control valve/solenoid and inspect its connector and wiring.
- Check the control circuit for a short to voltage or an open back to the ECM.
- Test the control valve/solenoid resistance against spec and replace if faulty.
- Repair the wiring or replace the valve, clear the code, and confirm normal vacuum-system operation.
FAQ
Quick answers
Is it safe to drive with P257C?+
Usually, but verify the brakes still feel normal. On vehicles where the reservoir supports brake-booster vacuum, a vacuum fault can reduce brake assist, so get it checked promptly.
Can I just clear P257C?+
You can, but a shorted circuit or bad control valve will set it again. Diagnose the wiring and valve first.
Related codes
Often seen with P257C
Read & clear P257C yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX