OBD-II code · Powertrain (generic, extended)
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.
Is it safe to drive with P21F4?
Usually safe to drive in the short term. Get it diagnosed when convenient, sooner if the car drives differently.
Common causes
- Most likelyVacuum leak
- PossiblePort injectors clogged
- PossibleFuel pressure low
Symptoms
- Check engine light on
- Rough idle, possible stalling
- Hesitation or hard cold start
- Power loss under load
Repair at a glance
- Difficulty
- Medium
- DIY friendly
- Often, with basic tools
- Typical labour
- 0.7–4 h
- Emissions test
- Likely to fail
- Parts involved
- Fuel Injector, Fuel Pressure Regulator, Oxygen Sensor
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.