P21F9: Fuel Control System B Too Rich at Idle (Bank 1)

P21F9 reports that bank 1's second fuel path runs rich at idle even though its correction has already reached the lower limit. No manufacturer documents this code publicly; Audi's 2.5 TFSI bulletin lists only the lean idle code P21F8 and the off-idle pair. Ford's port injection monitor shows how such a check works: it runs in closed loop with purge switched off, coolant at 70 to 110 °C and at least 12 % load, and flags a rich fault when long-term trim passes 25 % in the rich direction while short-term trim agrees, for 2 s (P2BF1, without separate idle cells). Typical causes at idle are a port injector that drips after closing, low-pressure regulation that is too high, or fuel vapour reaching the intake via crankcase ventilation. P2188 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
• Rough idle and fuel smell
• Higher consumption in town traffic

Most likely causes

• Port injector on bank 1 dripping after closing
• Low-pressure fuel regulation holding too much pressure
• Fuel vapour entering via crankcase ventilation (fuel-diluted oil)

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 P21F9 means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 1: Air and fuel
• F9: Specific fault in that system

Frequently asked questions

Can I keep driving with code P21F9?

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 P21F9?

The most common causes are: port injector on bank 1 dripping after closing; low-pressure fuel regulation holding too much pressure; fuel vapour entering via crankcase ventilation (fuel-diluted oil).

Related codes

• P21F8Fuel Control System B Too Lean at Idle (Bank 1)
• P21FBFuel Control System B Too Rich at Idle (Bank 2)
• P21F5Fuel Control System B Too Rich Off Idle (Bank 1)
• P21FDFuel Control System B Too Rich at Higher Load (Bank 1)
• P2188System Too Rich at Idle (Bank 1)
• P2BF1Fuel Control System B Too Rich Bank 1

Sources

• fordservicecontent.com: OBD Operations Summary Gasoline 2020 2023
• law.cornell.edu: 13 CCR 1968.2
• static.nhtsa.gov: MC 10174950 0001

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.