P21FC: Fuel Control System B Too Lean at Higher Load (Bank 1)

What does P21FC mean?

P21FC is the lean check of bank 1's second fuel path in the upper load zone, the third cell next to idle (P21F8) and part load (P21F4). Under high load, delivery capacity matters most: a weak lift pump, a clogged filter or a low-pressure rail that sags at high flow show up here first. On engines combining both paths, the share of each depends on the operating point; Audi's 2.5 TFSI, for example, uses 95 % direct and only 5 % manifold injection at heavy load during warm-up, so the second path's share and its trim learning vary strongly. No manufacturer documents P21FC publicly; Audi's bulletin for that engine covers only the idle and off-idle codes. Hesitation under full load is possible. P2191 is the system A counterpart.

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

Symptoms

• Check engine lamp on
• Hesitation or power loss at full load

Common causes

• High Weak in-tank lift pump or clogged fuel filter
• Medium Port rail pressure dropping at high flow
• Medium Port injectors on bank 1 flowing too little
• Low Charge air leak behind the air mass sensor

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