I responded to this same question at length receintly and you may find it with a search.
A couple of highlights.
Pre cats are part of the emissions system and tampering or removing is illegal.
OBD II systems (96 and newer) have a cat monitor built into the EEC system and will set the check engine light if the cat is removed or if it is not working well.
The cat monitor only monitors the pre cats on our cars. The readings on a smog test may be clean enough to otherwise pass, but the test would be failed from the CEL.
The CEL can be turned off momentarily by disconnecting the battery or removing the fuses to the EEC momory long enough for the memory to be dumped. It will stay off until the next time the cat monitor runs, which can be as soon as an hour or two of driving or as long as several weeks depending on when conditions are right for the cat monitor to run.
When the monitors have not been completed (there are several, not just the cat monitor) there is a code in the processorer that says it has not completed yet. In the near future, at least in California, if the code showing the monitors have not run is present, a smog test cannot be done.
There are several sources that sell devices that fool the EEC system into believing that the cat is OK. It will keep the monitor from detecting that the cat is not working. This is of course not legal. Another answer is that you can extend the wiring for the rear O2 sensors and mount them behind the main cat, but this isn't really legal either.
The main purpose of the pre cats is to clean up the exhaust more quickly after a cold start. The first two or three minutes are the most critical. They really are not needed much after that, that is why the tailpipe readings may be clean enough without the pre cats. Anyway, it is your choice and your risk.
