[Jpjr]

I don't think they would check the ECM for manipulation to replace the cats. I think they would only really check if you hurt your motor.

And on that note, if you did blow your motor they can sometimes tell if you have flashed to stock if they want to get forensic (which Ford does with all the Mustang guys that blow their motors). They can look to an abnormal number of battery disconnects or reflash points and some even have secret codes that are set off if the tune is changed.