P257B
Vacuum Reservoir Control Circuit Low
Vacuum reservoir control
Generic OBD-II
Medium
What it means
P257B means the control circuit for the vacuum reservoir is reading a voltage lower than expected. This circuit operates a solenoid that manages stored engine vacuum used by actuators — and on some vehicles the brake booster. A low reading usually points to a wiring short to ground or a faulty control solenoid. The effect depends on what the reservoir serves, so it is worth confirming before driving far.
Medium
Can I keep driving? Usually drivable, but if the affected system feeds the brake booster, pedal effort can rise — have it checked.
Common symptoms
What you'll notice
- Check-engine light on
- Vacuum-actuated device not operating
- Firmer brake pedal on systems where this supplies booster vacuum
- Often no obvious symptom otherwise
Common causes
Most likely first
- Short to ground in the control circuitLikely
- Faulty vacuum control solenoid or valveCommon
- Wiring or connector faultCommon
- ECM output-driver fault (rare)Less often
How to diagnose & fix
Work through it in order
- Check the control circuit for a short to ground.
- Inspect the vacuum control solenoid and its wiring and connector.
- Command the output with a scan tool and confirm it responds.
- Verify the vacuum supply and reservoir hold vacuum.
- Replace the solenoid if it fails testing.
FAQ
Quick answers
Is it safe to drive with P257B?+
Often yes, but confirm which system this vacuum reservoir serves. If it affects the brake booster, get it checked before driving far.
Can I just clear P257B?+
It returns while the circuit still reads low. Fix the solenoid or wiring first.
Related codes
Often seen with P257B
Read & clear P257B 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