Emissions fail due to NOx reading
 

Hey I have a 90 Camry V6 LE and i recently got from my dad and it failed the emissions test because it got a reading of 4 on the NOx. My question is that when my father just had it inspected in October it failed for the same reason and the garage said it was the catalitic convertor and they also cleaned the PCV valves. After that it passed inspection, now i tried to get it inspected today (1.18) it failed due to the NOx reading again. Could this be a falty catalitic convertor or could it possibly be anything else.... help please!!!