I did search and every thread has different info.
Some people were saying that it's caused due to misfires which are caused by a bad coil. I haven't noticed any misfires. Seems like the car is running fine. I'm at 147000 miles and I did change the plugs about 10,000 miles ago.
Other people said it was the pre-heater.
Some people were saying it was the rear O2 sensor going out.
I found a post by you specifically that states:
"Secondly, the code P0421 does not mean that your car is running rich - it means that your catalytic converter is bad. There's not really any other way to argue this, other than me pointing you to the page in the repair manual that directly states this."
Then your next post in that thread states if the cat went out something else IS wrong with the car.
So I guess if my car is having another issue I need to troubleshoot and fix that problem before replacing the cat or I'll ruin another one.
I guess I'll just start throwing money at the problem until it goes away.
That's a defeatist mentality. It is true that 99 times out of 100 a P0421 code means that your front cat is bad. It also means that 99 times out of 100, something else caused it to go bad. Are your ignition coils and spark plug wires the original ones on your car? I would say that every 90,000 miles is a good interval to change these.
Also check the tightness of your O2 sensors. If the back one goes bad it will not trigger this code, but if the front one goes bad, it will. I would replace your front O2 sensor every 90,000 miles as well, since it does wear out over time (only replace your front one, the rear one doesn't need to be changed) and can hurt your gas mileage.
Finally, if all else fails, you must have an exhaust leak at one of the flanges of your cat. This is highly unlikely, since the thing has so many bolts holding it, and there are no curves or anything in the cat itself that would cause sealing issues.
To check your coils and spark plug wires, refer to the pages in the shop manual for measuring the resistances of these items with a voltmeter. For the O2 sensor, you can heat the sensor end with a propane torch and measure the output with a voltmeter. As soon as you remove the flame the voltmeter should fall back down to near 0. You can also have a shop do these things as well, but personally, if I were anywhere near 90,000 miles, I'd just replace the ignition coils, wires, spark plugs, and front O2 sensor and be done with it. All of these items can be had cheaply on RockAuto.com
As for the cat, either replace it with a used OEM cat, a new Walker cat from RockAuto.com, or a header. If you feel like going ultra cheap, just do the O2 sensor non-fouler trick that is described in detail on this forum.
I apologize for coming off as short earlier, but this is really not a difficult problem to diagnose if you read the applicable pages in the repair manual. Trust me
