Failed smog check, hi NOx, no error lights - What to fix?

Great. Thanks very much.

Again, pardon the gaps in my mechanical knowledge – Which filters are you referring to here?

Would like to know if I can repair any parts of the EGR system myself – seen some videos, but not entirely clear to me yet.

Also, would you go with an OEM cat, or aftermarket?

By the way, timing belt has been replaced recently.

This was – is – my first car, so I’m finding out the hard way about maintenance schedules. What are the best book/film/online resources for taking on DIY procedures?

Keep in mind that the reason I don’t get tune-ups is that I don’t have any spare money to throw into the car.