NOx is through the roof - fromw hat I understand its caused by high cylinder temperatures.

Either the AFR is out, EGR is not functioning, or maybe fuel pressure is low/ filter dirty?
OR the CAT is bad.

Either way - its gonna need fixing sooner or later.

It'd make sense to start with a tune-up. You can go to autozone, get the parts and do it yourself or someone can do it for you.