What it means
Fuel control system B on bank 2 has detected a rich condition at idle. The code identifies the operating condition and fuel control system; it does not specify a particular cause.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Fuel control system B adjusts fueling for bank 2, the engine bank identified by the vehicle’s firing layout. The control module compares the bank’s mixture feedback with its target while the engine is idling; it stores P21FB when that feedback indicates too much fuel under that condition.
Common causes
Most likely first.
- Leaking or stuck-open injector on bank 2
- Excessive fuel pressure
- Faulty fuel pressure regulator
- Faulty air-fuel ratio or oxygen sensor
- Purge valve stuck open
Symptoms
- Check engine light on
- Increased fuel consumption
- Black smoke from exhaust on acceleration
- Smell of unburned fuel, fouled spark plugs
How to diagnose it
- Read freeze-frame data and fuel trims to confirm the fault occurred at idle and is concentrated on bank 2.
- Inspect bank 2 intake and exhaust connections, sensor wiring, and injector connectors for damage or leaks that could distort mixture feedback.
- Compare bank 2 air-fuel or oxygen sensor readings with bank 1 at warm idle to see whether the rich indication is isolated or shared.
- Check fuel pressure and regulator operation against the manufacturer’s specification, including whether pressure remains excessive at idle.
- Test bank 2 injectors for leakage or abnormal delivery, and check whether the purge valve allows vapor flow when it should be closed.
Common fixes
- Repair damaged wiring or poor connections at bank 2 mixture sensors or injectors.
- Replace a purge valve that leaks vapor at idle.
- Repair or replace a fuel pressure regulator or related pressure fault.
- Clean or replace a leaking or stuck-open bank 2 injector.
- Replace a faulty bank 2 air-fuel or oxygen sensor after confirming its signal is inaccurate.
Can I keep driving?
If the engine runs smoothly, limit driving and avoid hard acceleration; a rich mixture can waste fuel and damage the catalytic converter. Stop if it misfires badly or produces heavy black smoke.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.