Replace O2 sensors in pairs. Replace the PCV valve. Replace the fuel filter. Replace spark plugs and check timing. Clean the IAB valve. Basically do a complete tune-up. Use 87 octane gasoline. If it's something like a cat, you may be able to get a waiver. I think most states put a cap on how much you have to spend to fix the problem. It was $200 hear in Minnesota for 81 and newer cars. A new cat would exceed that.
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351W 89 Mustang GT Convertible