P21FD: Fuel Control System B Too Rich at Higher Load (Bank 1)

P21FD sets when the second fuel path on bank 1 is still rich at higher load although its learned correction has already taken away the permitted maximum. No manufacturer documents the code publicly. The FCA patent for port plus direct injection engines describes three separate monitors, one per injection mode, so a rich error that appears only while the port system fuels alone can be pinned on that system rather than reported as a general P0172. Possible causes are an air mass sensor reading high at large airflow, a regulator in the low-pressure circuit holding too much pressure, or port injectors that over-deliver at long opening times. Higher fuel consumption and sooty tailpipes may follow. P2192 is the system A counterpart, P21FF the bank 2 code.

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

Symptoms

• Check engine lamp on
• Increased consumption, sooty tailpipe

Most likely causes

• Air mass sensor reading too high at large airflow
• Low-pressure regulator holding too much pressure
• Port injectors on bank 1 over-delivering

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 P21FD means

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

Frequently asked questions

Can I keep driving with code P21FD?

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 P21FD?

The most common causes are: air mass sensor reading too high at large airflow; low-pressure regulator holding too much pressure; port injectors on bank 1 over-delivering.

Related codes

• P21FCFuel Control System B Too Lean at Higher Load (Bank 1)
• P21FFFuel Control System B Too Rich at Higher Load (Bank 2)
• P21F5Fuel Control System B Too Rich Off Idle (Bank 1)
• P21F9Fuel Control System B Too Rich at Idle (Bank 1)
• P2192System Too Rich at Higher Load (Bank 1)
• P2BF1Fuel Control System B Too Rich Bank 1

Sources

• patents.google.com: en
• 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.