P21A4Cylinder 9 Air-Fuel Ratio Imbalance
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- P21A4 is reserved for an imbalance at cylinder 9 and can therefore only appear on engines with ten or more cylinders, a small group of V10 and V12 petrol engines. No manufacturer documents the code publicly and no bulletin in the NHTSA index mentions it. CARB's rule (13 CCR 1968.2 (e)(6.2.1)(C)) still applies to these engines, and it allows a maker to store a bank code or even a misfire code instead when its hardware cannot single out a cylinder. With five or six cylinders feeding one front sensor, separating them needs extra effort, for example a frequency analysis tied to the engine cycle as in the Cummins patent, or an active test that changes the fuel of one cylinder at a time as VW does on its five-cylinder. Diagnosis follows the usual order: injector, intake sealing, compression of cylinder 9.
- What you may notice
- Check engine light on
- Possible P0309 with a large deviation
- Likely causes
- LikelyCylinder 9 injector clogged or leaking
- PossibleIntake leak at cylinder 9
- RareCompression loss at cylinder 9
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Hard · Usually a job for a workshop.Typical cost: €0 – €2,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.