P21F7: Fuel Control System B Too Rich Off Idle (Bank 2)

P21F7 sets when the second fuel path on bank 2 has to remove more fuel above idle than its rich limit permits. It needs an engine with two banks, separate front sensors and dual injection; inline engines such as Audi's 2.5 TFSI only have the bank 1 codes. No manufacturer publishes criteria for P21F7. An FCA patent for port plus direct injection engines explains the idea behind such split codes: a fault seen while only one path is fuelling is assigned to that path, whereas a fault during mixed operation goes to a general code such as P0175. When only this bank is affected, a leaking port injector on bank 2 or a bank 2 front sensor that reads rich is plausible; if both banks are rich, check what they share, such as pressure regulation. P2180 is the system A counterpart.

Bank 2 is the side without cylinder 1, so it only exists on V or boxer engines.

Symptoms

• Check engine lamp on
• Increased fuel consumption

Most likely causes

• Leaking port injector on bank 2
• Bank 2 front oxygen sensor reading rich
• Low-pressure regulation too high (if bank 1 also rich)

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 P21F7 means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 1: Air and fuel
• F7: Specific fault in that system

Frequently asked questions

Can I keep driving with code P21F7?

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 P21F7?

The most common causes are: leaking port injector on bank 2; bank 2 front oxygen sensor reading rich; low-pressure regulation too high (if bank 1 also rich).

Related codes

• P21F5Fuel Control System B Too Rich Off Idle (Bank 1)
• P21F6Fuel Control System B Too Lean Off Idle (Bank 2)
• P21FBFuel Control System B Too Rich at Idle (Bank 2)
• P21FFFuel Control System B Too Rich at Higher Load (Bank 2)
• P2180System Too Rich Off Idle (Bank 2)
• P2BF3Fuel Control System B Too Rich Bank 2

Sources

• patents.google.com: en
• fordservicecontent.com: OBD Operations Summary Gasoline 2020 2023
• 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.