Low or no power, no starts. Those Honeycombs start to break apart and start flowing downstream and clog and then ruin your nice Magnaflow or Flowmaster muffler(s). Cats themselves can melt inside and block flow. On other cars, you can remove the cats and put an X or Y Pipe but make sure you have a bung to plug in the O2 sensor. Smog time, put the cats back in.