Powertrain, generic (extended)

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

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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.

Likely causes

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

Symptoms

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

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€50–800
Time0.5–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

fuel injectorfuel pressure regulator

Related codes

P21F8P21FBP21F5P21FDP2188P2BF1

References: fordservicecontent.com, law.cornell.edu, static.nhtsa.gov.