Powertrain, generic (extended)

P21F4 Fuel Control System B Too Lean Off Idle (Bank 1)

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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.

Likely causes

Outdated engine control software (Audi 2.5 TFSI, bulletin 2059429/1)
Port injectors of bank 1 restricted or delivering too little
Pressure in the low-pressure (port) fuel rail too low
Air mass sensor reading too low

Symptoms

  • Check engine lamp on
  • Weak performance or hesitation at part load
  • Often stored together with P2177 or P0087

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

P21F5P21F6P21F8P21FCP2177P2BF0

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