P21F4 — Fuel Control System B Too Lean Off Idle (Bank 1)
The secondary fuel control loop B on bank 1 reports a lean condition outside idle. On dual-injection systems (port + direct) the secondary loop trims port injection independently from the main loop.
General code reference. Confirm the definition and diagnostic procedure for your vehicle's year, model and engine. A stored code alone does not identify a failed part.
Affected components
- Fuel injector
- Fuel pressure regulator
- Oxygen sensor
Possible causes to check
These are possible checks, not a ranked diagnosis. Follow the vehicle manufacturer's test procedure before replacing parts.
- Vacuum leak
- Port injectors clogged
- Fuel pressure low
Possible symptoms
- Check engine light on
- Rough idle, possible stalling
- Hesitation or hard cold start
- Power loss under load
Diagnostic notes
- The malfunction indicator lamp may illuminate
- This code can affect emissions
Related codes
Sources
Check each reference's vehicle and model-year scope. General background links do not establish which fault is most likely on your vehicle.
Diagnostic information is provided for reference only. Confirm the code with a compatible scan tool and follow the vehicle manufacturer's service procedures before replacing components.