P21F8Fuel Control System B Too Lean 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
- P21F8 covers the idle zone: the second fuel path of bank 1 needs more extra fuel at idle than its adaptation allows. Airflow at idle is so small that a fixed amount of unmetered air or a slightly sluggish injector shifts the mixture a lot, which is why makers learn idle separately from part load. Audi is the only manufacturer naming the code publicly: bulletin 2059429/1 of May 2020 places it among the trim and fuel pressure codes of the 2017 to 2018 RS 3 and TT RS with the 2.5 TFSI and resolves them with new ECM software, stating that replaced parts are not reimbursed. On other engines, check for intake leaks behind the air mass sensor, a port injector delivering too little, or low pressure in the port fuel rail. P2187 is the matching code for system A.
- What you may notice
- Check engine lamp on
- Unsteady or hunting idle
- Likely causes
- LikelyOutdated engine control software (Audi 2.5 TFSI, bulletin 2059429/1)
- PossibleUnmetered air entering behind the air mass sensor
- PossiblePort injector on bank 1 delivering too little
- RareLow pressure in the port fuel rail
- 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: €0 – €700
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.