P21F5Fuel Control System B Too Rich Off 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
- P21F5 is the rich limit for the second fuel path of bank 1 in the part-load zone above idle: the learned correction already removes as much fuel as allowed, yet the front sensor still sees surplus fuel. Audi lists the code for the 2.5 TFSI in the 2017 to 2018 RS 3 and TT RS, whose manifold injectors run on a 7 bar rail beside the high-pressure system, and cures it by reprogramming the engine control module (bulletin 2059429/1). In other cases, a port injector that leaks or opens too long, excessive pressure in the low-pressure rail or an air mass reading that is too high are typical. An FCA patent for such engines clears a mode-specific fault only after the monitor passes again in the same injection mode. Expect higher consumption; P2178 is the system A partner.
- What you may notice
- Check engine lamp on
- Increased fuel consumption
- Fuel smell from the exhaust
- Likely causes
- LikelyOutdated engine control software (Audi 2.5 TFSI, bulletin 2059429/1)
- PossiblePort injector on bank 1 leaking or sticking open
- RarePressure in the low-pressure rail above specification
- RareAir mass sensor reading too high
- 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 – €800
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.