P21FB: Fuel Control System B Too Rich at Idle (Bank 2)

P21FB means bank 2's second fuel path is rich at idle while its correction already sits at the rich limit. The code requires an engine with a separate front sensor per bank and two injection paths; no maker documents it publicly. CARB section (e)(6.2.2) is the legal basis: once adaptive fuel control has used up the adjustment the manufacturer allows, a fault must be stored. Rich idle on one bank only typically comes from a port injector on that bank that leaks or sticks open, or from a bank 2 front sensor that reports too rich. If both banks show rich idle codes, look at shared items such as low-pressure regulation or fuel entering through crankcase ventilation. Fuel smell and a rough idle are possible; P2190 is the system A sibling.

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

Symptoms

• Check engine lamp on
• Rough idle and fuel smell

Most likely causes

• Port injector on bank 2 leaking or sticking open
• Bank 2 front oxygen sensor reading rich
• Low-pressure regulation too high or fuel from crankcase ventilation (if both banks 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 P21FB means

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

Frequently asked questions

Can I keep driving with code P21FB?

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 P21FB?

The most common causes are: port injector on bank 2 leaking or sticking open; bank 2 front oxygen sensor reading rich; low-pressure regulation too high or fuel from crankcase ventilation (if both banks rich).

Related codes

• P21FAFuel Control System B Too Lean at Idle (Bank 2)
• P21F9Fuel Control System B Too Rich at Idle (Bank 1)
• P21F7Fuel Control System B Too Rich Off Idle (Bank 2)
• P21FFFuel Control System B Too Rich at Higher Load (Bank 2)
• P2190System Too Rich at Idle (Bank 2)
• P2BF3Fuel Control System B Too Rich Bank 2

Sources

• law.cornell.edu: 13 CCR 1968.2
• fordservicecontent.com: OBD Operations Summary Gasoline 2020 2023

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.