Is high NOx the only problem?
Lean air-fuel ratio results in high cylinder temperature and excessive NOx and high HC, high O2, and low CO and CO2 emissions….
A lean air-fuel ratio may be caused by low fuel pump pressure, partially plugged injectors, a vacuum leak, or defective O2, MAP, ECT, or IAT sensors. Could also be that a catalytic converter is either damaged or plugged...and of course any combination of all suspects.
Read the codes off the computer to start.