when my TT was running hot after i bought her, i also failed emissions because my NOx was too high (allowable was 791 or so, i had a reading of 963)...when i replaced the fan clutch, the water temp came down A LOT and that fixed my heating problem completely...then i went back to try again and my NOx was ONLY 157 with a properly cooling TT...i think running hot causes the EGR to basically become useless because you're putting the hot gases back into the car...i dont know the entire technical reason, someone please feel free to step in and explain it better or correct me...but yes, that was the reason my NOx was too high...i used datascan to check my temps