I am led to believe that it is only disabled for the rear sensors or cat issues, however Mark will be able to clerify.
+1 - just disables those sensors, CEL will still come on if there are other problems. Not sure how they do it in the software, but the easiest way for them would just to allow the range of values before CEL to be very wide. That way they arent changing a whole load of code/logic, just letting that sensor be any value without throwing the CEL.