P221B code meaning
NOx Sensor 1/2 Correlation (Bank 2)
Powertrain system Generic code P22xx codes Check engine light
What P221B means
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.
P221B at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyHard
- Typical labor1 to 4 hours
- Clears after repairAfter a drive cycle
Symptoms of P221B
- Malfunction indicator lamp on
- Often stored with SCR efficiency codes
- Inlet and outlet NOx values for bank 2 disagree in live data
Common causes of P221B
- Most likelyOne of the two bank 2 NOx sensors reading wrongly
- PossibleExhaust leak between the two sensors
- Less likelyDosing that does not stop or leaks, distorting the comparison
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Nox sensor
What each part of P221B means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 2 | Generic code (second SAE range): it means the same on every make. |
| 2 | Area: fuel and air metering (injector circuit). |
| 1B | The exact fault in that area. |
What to do next
- Note P221B 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 P221B
What does P221B mean?
P221B means "NOx Sensor 1/2 Correlation (Bank 2)". P221B compares the NOx sensors before and after the SCR catalyst of bank 2. It is a generic code, so it means the same on every make.
Is it safe to drive with P221B?
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 P221B fail an emissions test?
Yes, it can. P221B 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 P221B to fix?
Difficulty: hard. A typical repair takes 1 to 4 hours of work. This is a job for a workshop.
Will P221B 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.