Powertrain, generic (extended)

P21F5 Fuel Control System B Too Rich Off Idle (Bank 1)

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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.

Likely causes

Outdated engine control software (Audi 2.5 TFSI, bulletin 2059429/1)
Port injector on bank 1 leaking or sticking open
Pressure in the low-pressure rail above specification
Air mass sensor reading too high

Symptoms

  • Check engine lamp on
  • Increased fuel consumption
  • Fuel smell from the exhaust

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€0–800
Time0.5–3 h

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

Parts involved

fuel injectorfuel pressure regulatoroxygen sensor

Related codes

P21F4P21F7P21F9P21FDP2178P2BF1

References: static.nhtsa.gov, static.nhtsa.gov, patents.google.com, law.cornell.edu.