A code scanner could help, but it may also be helpful if you include more information about your car. What test did you fail and by how much? What modifications do you have? New heads? Timing? What gasoline do you use? Etc. Give us some background and maybe we can help.
My '89 with a 351W and hi-flow cats passes with better scores than my wife's '91 V6 Probe.
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351W 89 Mustang GT Convertible