P21F5: Fuel Control System B Too Rich Off Idle (Bank 1)

What it means

Fuel control system B on bank 1 has detected a rich condition off idle. The code identifies the operating condition and fuel control system; it does not specify a particular injection type or cause.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

Fuel control system B adjusts fueling for bank 1 using air-fuel and oxygen sensor feedback. This code sets when bank 1 is detected running rich while the engine is off idle, meaning the system sees excess fuel or insufficient measured air in that operating range.

Common causes

Most likely first.

1. Port injectors leaking
2. Fuel pressure high

Symptoms

• Check engine light on
• Increased fuel consumption
• Black smoke from exhaust on acceleration
• Smell of unburned fuel, fouled spark plugs

How to diagnose it

1. Save freeze-frame data and check bank 1 fuel trims and sensor readings at the off-idle speed and load recorded when the code set.
2. Inspect bank 1 intake and air-measurement plumbing for restrictions or disconnected parts that could make incoming air appear lower than it is.
3. Check for signs of fuel leakage at the port injectors and compare bank 1 spark plugs for wetness or heavy deposits.
4. Measure fuel pressure against the manufacturer's specification and investigate excessive pressure that could overfuel the bank.
5. If pressure and injectors appear normal, verify bank 1 sensor response and related wiring using manufacturer diagnostic information.

Common fixes

• Repair an intake restriction or disconnected air-measurement hose affecting the engine.
• Repair a leaking port injector on bank 1 if confirmed by testing.
• Correct excessive fuel pressure by repairing the identified pressure-control fault.
• Repair sensor wiring or replace a sensor only when its response test confirms a fault.

Can I keep driving?

Avoid extended driving, hard acceleration, and towing while the engine is running rich. Excess fuel can foul spark plugs and overheat the catalytic converter; stop if there is heavy smoke, severe misfiring, or a strong fuel smell.