P21A1Cylinder 6 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
- P21A1 marks cylinder 6, the last cylinder of an inline six or V6. The clearest example comes from the 2025 RAM 1500 with the 3.0 Hurricane twin-turbo inline six: the charge-air cooler housing had a loose crimp that let boost escape, the cylinders filled unevenly and P21A1 set together with other cylinder codes, accompanied by a whistle and lower boost; the cooler was inspected and replaced if needed. Toyota listed P219A, P219B and P219C to P21A1 for the 2015 Tacoma 1GR-FE and worked through a leak-down test to a valve job. Kia replaced high-pressure injectors and fuel lines on the 2022 Carnival 3.5 GDI, while Audi corrected a software setting on 2010 to 2012 3.0 TFSI models. A code for the last cylinder alone is no reason to suspect sensor or ECM before injector and compression are checked.
- What you may notice
- Check engine light on
- Whistling noise and reduced boost with a charge-air leak
- Possible P0306 or rough running
- Likely causes
- PossibleCharge-air cooler or boost leak (turbo engines, e.g. RAM 3.0 Hurricane)
- PossibleFaulty injector at cylinder 6
- PossibleValve leakage at cylinder 6
- RareECM software setting (e.g. Audi 3.0 TFSI)
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · 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.