P221A code meaning
NOx Sensor 1/2 Correlation (Bank 1)
Powertrain system Generic code P22xx codes Check engine light
What P221A means
P221A compares the two NOx sensors of bank 1, upstream and downstream of the SCR catalyst. When conditions should make both read alike, the gap between them is too large, so the system cannot tell which value to trust. Volvo and Mack list P221A among codes that often lead to needless sensor swaps; before replacement a NOx conversion test must show the inlet and outlet readings disagreeing by more than about 40 ppm shortly before engine shutdown, or the cause lies elsewhere. Nissan bulletin NTB16-048 covers early 2016 Titan XD trucks with the Cummins 5.0L V8: P221A with P229E and P22A7 is fixed by a new outlet sensor and ECM reprogramming. P2201 and P229F judge each sensor alone, P221B is bank 2.
P221A 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 P221A
- Malfunction indicator lamp on
- Often stored with P20EE or other SCR codes
- Implausible NOx conversion values in live data
Common causes of P221A
- Most likelyOne of the two NOx sensors (often the outlet sensor) reading wrongly
- PossibleExhaust leak between the two sensors diluting the reading
- PossibleECM software (Nissan Titan XD 5.0L 2016, NTB16-048)
- Less likelyFaulty DEF dosing 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 P221A 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). |
| 1A | The exact fault in that area. |
What to do next
- Note P221A 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 P221A
What does P221A mean?
P221A means "NOx Sensor 1/2 Correlation (Bank 1)". P221A compares the two NOx sensors of bank 1, upstream and downstream of the SCR catalyst. It is a generic code, so it means the same on every make.
Is it safe to drive with P221A?
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 P221A fail an emissions test?
Yes, it can. P221A 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 P221A to fix?
Difficulty: hard. A typical repair takes 1 to 4 hours of work. This is a job for a workshop.
Will P221A 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.