Powertrain, generic (extended)

P221A NOx Sensor 1/2 Correlation (Bank 1)

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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.

Likely causes

One of the two NOx sensors (often the outlet sensor) reading wrongly
Exhaust leak between the two sensors diluting the reading
ECM software (Nissan Titan XD 5.0L 2016, NTB16-048)
Faulty DEF dosing distorting the comparison

Symptoms

  • Malfunction indicator lamp on
  • Often stored with P20EE or other SCR codes
  • Implausible NOx conversion values in live data

Repair estimate

DifficultyHardWorkshop job
Parts and labour€100–1,500
Time1–4 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

nox sensor

Related codes

P221BP2201P229FP229EP22A7P20EE

References: static.nhtsa.gov, static.nhtsa.gov, static.nhtsa.gov, law.cornell.edu.