Powertrain, generic (extended)

P21F8 Fuel Control System B Too Lean at Idle (Bank 1)

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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.

Likely causes

Outdated engine control software (Audi 2.5 TFSI, bulletin 2059429/1)
Unmetered air entering behind the air mass sensor
Port injector on bank 1 delivering too little
Low pressure in the port fuel rail

Symptoms

  • Check engine lamp on
  • Unsteady or hunting idle

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€0–700
Time0.5–3 h

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

Parts involved

fuel injectorintake manifoldoxygen sensor

Related codes

P21F9P21FAP21F4P21FCP2187P2BF0

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