P21F9Fuel Control System B Too Rich at Idle (Bank 1)
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- P21F9 reports that bank 1's second fuel path runs rich at idle even though its correction has already reached the lower limit. No manufacturer documents this code publicly; Audi's 2.5 TFSI bulletin lists only the lean idle code P21F8 and the off-idle pair. Ford's port injection monitor shows how such a check works: it runs in closed loop with purge switched off, coolant at 70 to 110 °C and at least 12 % load, and flags a rich fault when long-term trim passes 25 % in the rich direction while short-term trim agrees, for 2 s (P2BF1, without separate idle cells). Typical causes at idle are a port injector that drips after closing, low-pressure regulation that is too high, or fuel vapour reaching the intake via crankcase ventilation. P2188 is the system A partner.
- What you may notice
- Check engine lamp on
- Rough idle and fuel smell
- Higher consumption in town traffic
- Likely causes
- PossiblePort injector on bank 1 dripping after closing
- PossibleLow-pressure fuel regulation holding too much pressure
- RareFuel vapour entering via crankcase ventilation (fuel-diluted oil)
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €50 – €800
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.