P21F9: Fuel Control System B Too Rich at Idle (Bank 1)

What does P21F9 mean?

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.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

• Check engine lamp on
• Rough idle and fuel smell
• Higher consumption in town traffic

Common causes

• Medium Port injector on bank 1 dripping after closing
• Medium Low-pressure fuel regulation holding too much pressure
• Low Fuel vapour entering via crankcase ventilation (fuel-diluted oil)

Likelihood is a general estimate across vehicles; your make and model may differ.