P21F4Fuel Control System B Too Lean 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
- P21F4 reports that the second fuel path on bank 1 needs more added fuel than its adaptation may supply once the engine has left idle, in the part-load range. Engines with port and direct injection learn a correction for each path; Ford calls its direct system A and its port system B. Audi documents the code on the 2017 to 2018 RS 3 and TT RS with the 2.5 TFSI five-cylinder (DAZA), which pairs 250 bar direct with 7 bar manifold injection: bulletin 2059429/1 groups it with P21F5, P21F8, P2177, P2187 and P0087, reports MIL and weak performance, and prescribes ECM software 0005 instead of new parts. Without such a bulletin, suspect restricted port injectors or too little pressure in the low-pressure rail. The system A counterpart is P2177.
- What you may notice
- Check engine lamp on
- Weak performance or hesitation at part load
- Often stored together with P2177 or P0087
- Likely causes
- LikelyOutdated engine control software (Audi 2.5 TFSI, bulletin 2059429/1)
- PossiblePort injectors of bank 1 restricted or delivering too little
- PossiblePressure in the low-pressure (port) fuel rail too low
- RareAir mass sensor reading too low
- 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.