P221BNOx Sensor 1/2 Correlation (Bank 2)
- 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
- P221B compares the NOx sensors before and after the SCR catalyst of bank 2. When operating conditions should make them agree, their readings differ by more than the calibration allows, and the controller cannot decide which value is wrong. The code requires a separate SCR line on bank 2, which no publicly documented diesel has; no manufacturer lists P221B. The bank 1 twin P221A shows the practice: Volvo and Mack require a NOx conversion test before any part is changed and only accept a sensor fault if inlet and outlet readings differ by more than roughly 40 ppm shortly before shutdown, while Nissan bulletin NTB16-048 replaced the outlet sensor and reflashed the ECM on 2016 Titan XD trucks.
- What you may notice
- Malfunction indicator lamp on
- Often stored with SCR efficiency codes
- Inlet and outlet NOx values for bank 2 disagree in live data
- Likely causes
- LikelyOne of the two bank 2 NOx sensors reading wrongly
- PossibleExhaust leak between the two sensors
- RareDosing that does not stop or leaks, distorting the comparison
- 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: €100 – €1,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.