P21F4 code meaning
Fuel Control System B Too Lean Off Idle (Bank 1)
Powertrain system Generic code P21xx codes Check engine light
What P21F4 means
P21F4 reports that the second fuel path on bank 1 needs more added fuel than its adaptation may supply once the engine has left idle, in the part-load range. Engines with port and direct injection learn a correction for each path; Ford calls its direct system A and its port system B. Audi documents the code on the 2017 to 2018 RS 3 and TT RS with the 2.5 TFSI five-cylinder (DAZA), which pairs 250 bar direct with 7 bar manifold injection: bulletin 2059429/1 groups it with P21F5, P21F8, P2177, P2187 and P0087, reports MIL and weak performance, and prescribes ECM software 0005 instead of new parts. Without such a bulletin, suspect restricted port injectors or too little pressure in the low-pressure rail. The system A counterpart is P2177.
P21F4 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium
- Typical labor0.5 to 3 hours
- Clears after repairAfter a drive cycle
Symptoms of P21F4
- Check engine lamp on
- Weak performance or hesitation at part load
- Often stored together with P2177 or P0087
Common causes of P21F4
- Most likelyOutdated engine control software (Audi 2.5 TFSI, bulletin 2059429/1)
- PossiblePort injectors of bank 1 restricted or delivering too little
- PossiblePressure in the low-pressure (port) fuel rail too low
- Less likelyAir mass sensor reading too low
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Fuel injectorFuel pressure regulatorOxygen sensor
What each part of P21F4 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 2 | Generic code (second SAE range): it means the same on every make. |
| 1 | Area: fuel and air metering. |
| F4 | The exact fault in that area. |
What to do next
- Note P21F4 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P21F4
What does P21F4 mean?
P21F4 means "Fuel Control System B Too Lean Off Idle (Bank 1)". P21F4 reports that the second fuel path on bank 1 needs more added fuel than its adaptation may supply once the engine has left idle, in the part-load range. It is a generic code, so it means the same on every make.
Is it safe to drive with P21F4?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P21F4 fail an emissions test?
Yes, it can. P21F4 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P21F4 to fix?
Difficulty: medium. A typical repair takes 0.5 to 3 hours of work. This is a job for a workshop.
Will P21F4 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.