My car has been tuned with HPTuners, the code will not post or even register there is no cat, also killed the stupid 108mph speed limiter as it is now set to 255mph and a few other transmission things have been changed.I had to replace the cats on my 97 Yukon. My son put some racine fuel in the tank and ruined them and the O2 sensors. The cats have to be in the exhaust as there are 2 oxygen sensors per exhaust. 1 in front and 1 behind the cat. If the cat is not there the computer will not run the engine management (fuel/air) properly and the mileage tanks. Replacement is required at least on GM's