P25FB is set when the control circuit of bypass valve C is not open or shorted, but its electrical behaviour does not match the command – for instance a current that rises too slowly or settles at the wrong level, or a signal that drops out now and then. That points to a coil whose resistance has changed with heat, contact resistance in the connector, or a valve whose armature moves sluggishly and so alters the current curve. Pure circuit faults are reported as P25F8 to P25FA, a valve that is mechanically stuck as P25FC. Which compressor valve C belongs to is decided by the manufacturer, and none documents this code publicly; CARB requires output components to be checked for proper function. Consequences range from surging on lift-off to reduced boost.
Medium severity. Can I keep driving?Drive without demanding power and go to a repair shop soon. If you see a lot of blue smoke or hear a metallic noise, shut off the engine.
Symptoms
- Check engine lamp on
- Occasional fluttering on lift-off
- Uneven boost build-up
Most likely causes
- Valve coil resistance changed by heat ageing
- Contact resistance or moisture in the valve connector
- Sluggish valve armature due to deposits
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 P25FB means
- P: Powertrain: engine and transmission
- 2: Generic code: same on every make
- 5: Speed and idle
- FB: Specific fault in that system
Frequently asked questions
Can I keep driving with code P25FB?
Drive without demanding power and go to a repair shop soon. If you see a lot of blue smoke or hear a metallic noise, shut off the engine.
What causes code P25FB?
The most common causes are: valve coil resistance changed by heat ageing; contact resistance or moisture in the valve connector; sluggish valve armature due to deposits.
Related codes
- P25F8Turbocharger/Supercharger Bypass Valve C Control Circuit
- P25F9Turbocharger/Supercharger Bypass Valve C Control Circuit Low
- P25FATurbocharger/Supercharger Bypass Valve C Control Circuit High
- P25FCTurbocharger/Supercharger Bypass Valve C - Mechanical
- P0039Turbocharger/Supercharger Bypass Valve A Control Circuit Range/Performance
- P00C3Turbocharger/Supercharger Bypass Valve B Control Circuit Range/Performance
Sources
- law.cornell.edu: 13 CCR 1968.2
- ms-motorservice.com: electric recirculating air valves
- en.wikipedia.org: Blowoff valve
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.