The most typical reasons for high NOx emissions are reduced EGR flow and lean mixture.
Reduced EGR flow is often corrected by cleaning the buildup from the EGR passages in the upper intake (behind the throttle body gasket).
A lean condition often happens over time as the MAF element gets dirty from dust in the incoming air. Try cleaning it. Air leaks in the intake system are fairly common but usually cause driveability problems.
Consult the FAQ or search for more info on these cleaning procedures.
