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1995 Volvo 940 4 cyl Two Wheel Drive Automatic 164000 miles
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.