Yeah, mine just kicked the code a few days ago.
Was gonna just weld on a straight pipe but found out if I did that the o2 sensor would kick a constant code on the dash cluster. Don't wanna deal with that annoying shit.
“MIL Eliminator”
Illegal, of course, but if you can get away with it…
A modern vehicle has at least two Oxygen sensors, one before the catalytic converter, and one after.
The one before is used to measure the amount of oxygen in the exhaust coming out of the engine, in order to adjust the fuel/air mixture. That's crucial to the proper operation of the engine. Too much oxygen means the engine is running lean, and needs more fuel for the amount of air that it's taking in, and not enough oxygen means it's too rich, and needs less fuel.
The one after the cat is just to monitor the cat itself. Of the oxygen remaining in the exhaust before it hist the cat, some of it is used to further react certain pollutants in the exhaust, so the exhaust coming out of the cat should have less oxygen than the exhaust had going into it. The readings from the two oxygen sensors are compared to verify that the cat is using as much oxygen as it should be to reduce pollutants. The P0420 and P0430 codes indicate that a catalytic converter is not using enough oxygen, meaning that it's probably not doing its job.
An MIL eliminator generates a fakes signal, simulating that from a post-cat oxygen sensor reading the output from a properly-working cat.
Now, whether you can get away with it is another thing.
The 1997 V6 Ford Contour that I used to have had a total of three cats. There was one coming off of each bank, built onto the exhaust manifolds, which fed into a Y-pipe that fed into a third catalytic converter. It was the first two, the one off each bank, that has O2 sensors set up to check them. When those cats failed, I installed MIL eliminators. As the exhaust still went through that third cat, my actual emissions were low enough to pass a smog test.
If the only cats in your car are the ones that have the O2 sensors set up to monitor them, then if they are failing, you probably won't be able to pass smog, as my Contour was able to do.