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

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.

Bank 1 is the side of the engine with cylinder 1. Inline engines only have this bank.

Symptoms

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

Most 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• Outdated engine control software (Audi 2.5 TFSI, bulletin 2059429/1)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

What to check, in order

Start with the simplest. You can tick off each step.

1. Check short-term and long-term fuel trims in live data
2. Look for air leaks at hoses and gaskets
3. Check the mass air flow sensor (MAF) and, if appropriate, clean it with a dedicated cleaner
4. Measure the fuel pressure
5. Check the injectors and the oxygen sensor

Good to know

• Repair difficulty: medium
• It affects emissions.

What each character of P21F5 means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 1: Air and fuel
• F5: Specific fault in that system

Frequently asked questions

Can I keep driving with code P21F5?

You can drive in moderation for a short time. A very lean or very rich mixture can damage the engine or the catalytic converter.

What causes code P21F5?

The most common causes are: outdated engine control software (Audi 2.5 TFSI, bulletin 2059429/1); port injector on bank 1 leaking or sticking open; pressure in the low-pressure rail above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.

Related codes

• P21F4Fuel Control System B Too Lean Off Idle (Bank 1)
• P21F7Fuel Control System B Too Rich Off Idle (Bank 2)
• P21F9Fuel Control System B Too Rich at Idle (Bank 1)
• P21FDFuel Control System B Too Rich at Higher Load (Bank 1)
• P2178System Too Rich Off Idle (Bank 1)
• P2BF1Fuel Control System B Too Rich Bank 1

Sources

• static.nhtsa.gov: MC 10174950 0001
• static.nhtsa.gov: MC 10127901 9999
• patents.google.com: en
• law.cornell.edu: 13 CCR 1968.2

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.