P21F8: Fuel Control System B Too Lean at Idle (Bank 1)

What it means

Fuel control system B on bank 1 has detected a lean condition at idle. The code identifies the operating condition and fuel control system; it does not specify a particular 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 sensor feedback, including while the engine idles. This code sets when bank 1 is detected lean at idle, when extra air entering through a vacuum, PCV, intake gasket, or purge path can have a large effect on the mixture.

Common causes

Most likely first.

1. Vacuum leak
2. PCV system leak or fault
3. Intake manifold gasket leak
4. Purge valve stuck open
5. Faulty air-fuel ratio or oxygen sensor

Symptoms

• Check engine light on
• Rough idle, possible stalling
• Hesitation or hard cold start
• Power loss under load

How to diagnose it

1. Save freeze-frame data and examine bank 1 fuel trims and air-fuel or oxygen sensor data at idle before changing parts.
2. Listen and inspect around bank 1 intake connections, vacuum hoses, and the intake manifold gasket for leaks that are strongest at idle.
3. Check PCV hoses and valve operation for a leak, then test whether the purge valve is allowing unintended flow at idle.
4. Compare bank 1 and bank 2 fuel trims at idle and at a raised engine speed to help distinguish an idle air leak from a broader fuel problem.
5. If no air leak is found, verify bank 1 sensor response and wiring using the manufacturer's diagnostic procedure.

Common fixes

• Replace a split or disconnected vacuum or PCV hose causing an idle leak.
• Repair a leaking bank 1 intake manifold gasket.
• Repair or replace a purge valve that is leaking when it should be closed.
• Repair sensor wiring or replace a sensor only if its response test shows a fault.

Can I keep driving?

Avoid driving if the engine stalls or cannot maintain idle, especially in traffic. If it remains stable, use a short gentle trip for diagnosis; prolonged lean running can cause misfires and excess engine heat.