I have a 1995 volvo,mileage 164000 kms.It failed the emission twice with the nox reading 2390 first,then 1540 at the second.After it failed the first time I had a major tuneup done-filters, plugs,wires,distr.cap.It runs good stutters a little sometimes.Please tell me what may be the reason why my car is failing this test.I took it for a highway drive for an hour and a half before testing.I use only supreme gas,oct91.Can this be the EGR valve or the catalytic converter?How can this be diagnosed?Mechanics seem to only want my money,not solve the problem.