NOx emissions are caused by excessive heat in the combustion chamber.
EGR is the primary emissions component that deals with NOx. You can check your EGR valve by applying vacuum to it while idling and your car should stall or start running poorly. If it doesn't, that means your EGR valve isn't opening when it should, which would lead to higher combustion chamber temps. A code would likely be generated for this. The EGR vacuum control solenoid can also be bad, but again, a code will be generated for that.
You need air being pumped to the cats for the cats to work. Are you sure your air tube is connected and that your air pump is pumping air?
You may be running too lean because your O2 sensors are starting to fail. I'd replace them if you haven't recently. Make sure your timing is set at stock and if it is already, don't waste your money on 94 octane gasoline.
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351W 89 Mustang GT Convertible