P21FA: Fuel Control System B Too Lean at Idle (Bank 2)

P21FA is the bank 2 version of the idle lean check for the second fuel path. It applies to engines with two cylinder banks, a front oxygen sensor per bank and dual injection, such as Ford's 5.0 V8, which has combined port and direct injection since 2018. No manufacturer publishes this code; Ford monitors its port system per bank without separate idle cells and would store the fault as P2BF2. At idle the per-bank correction reacts strongly to small absolute errors, so a leak at bank 2's intake runners or gasket, or a partly blocked port injector on that side, are the first suspects while bank 1 stays clean. Drivers may notice an uneven idle or nothing beyond the warning lamp. The system A equivalent is P2189.

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

Symptoms

• Check engine lamp on
• Uneven idle

Most likely causes

• Intake leak at bank 2 runners or gasket
• Partly blocked port injector on bank 2
• Bank 2 front oxygen sensor reading lean

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 P21FA means

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

Frequently asked questions

Can I keep driving with code P21FA?

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 P21FA?

The most common causes are: intake leak at bank 2 runners or gasket; partly blocked port injector on bank 2; bank 2 front oxygen sensor reading lean.

Related codes

• P21FBFuel Control System B Too Rich at Idle (Bank 2)
• P21F8Fuel Control System B Too Lean at Idle (Bank 1)
• P21F6Fuel Control System B Too Lean Off Idle (Bank 2)
• P21FEFuel Control System B Too Lean at Higher Load (Bank 2)
• P2189System Too Lean at Idle (Bank 2)
• P2BF2Fuel Control System B Too Lean Bank 2

Sources

• fordservicecontent.com: OBD Operations Summary Gasoline 2020 2023
• en.wikipedia.org: Ford Modular engine
• 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.