New to this board. Been researching reasons for backfire on my 05GT. Great info as to what to check. I do not have all the specs about the tune on car I would like to have as I just bought it in Feb. , but it has Ford Racing cai ,BBK short headers&crossover pipe, Pypes axelback muffler deletes. Car ran perfect for about 2 weeks then set a code for Cats. but still ran perfect , about 3 weeks ago I noticed a slight backfire randomly that has gotten worse . It is random that sometimes nothing then it will run like crap for a day then good the next. I saw a lot about the alt. causing a problem for some but just what / how can that be ? I only get the one code for cats and when I clear it it makes no diff. and resets in about 2 weeks. Can bad cats. cause this backfire? I do not know that much about these later ( post carb. ) cars. Thanks for any input you may have to give. Car still runs great otherwise.