Catalytic converter keeps failing

I have a 2002 EP3, and I can't get a catalytic converter to last longer than 2 years in it. 3 non-factory cats burned through in 6 mos-2 yrs each. Latest was a Magnaflow OEM replacement P/N 49142 which is now gone in about 2 yrs.

- All original K20A3, no performance-adders. Car is stock (or stock equivalent) except for 17" wheels.
- Just shy of 255,000 miles (1 owner).
- CEL code P0420, I also have CEL P1147 (I think, don't have OBDII tool handy) for an evap can, unsure if related. Failed CT emissions in June (fails on the CEL). Light came on in about... April?
- Compression is good across all four cylinders (above 135 psi and within 25 psi delta, tested 2 weeks ago)
1 - 184 psi
2 - 175 psi
3 - 167 psi
4 - 181 psi
- Average about 5000-6000 between oil changes, regular 5W-20 for both changes & top-ups.
- None of the spark plugs are fouled with oil, corroded from antifreeze, burnt, or appear anything but normal (changed 3 or 4 times over life of car & never fouled). Coils are original.
- Fuel mileage has seemingly dropped a bit, but before the CEL came on. (Injectors? O2 sensor? Running rich?)
- It does seem to stumble a little bit on acceleration (but I've had the A/C on all summer so it could be that). Does the CEL trigger a 'safe mode' that fiddles with the timing?
- Burns about 1qt of oil every 2000-2500mi or so.
- No known leaks/smells/smoke/discoloration.

Could it be an O2 sensor or exhaust leak causing a bad reading & rich condition? Could it be lean & overheated? I'm just not sure at this point.

I want to keep this car but I can't keep putting catalytic converters in it every 2 years.

I don't post much, so thanks in advance for any advice.

Even though I am a total n00b to this forum and just picked up one of these cars myself, I feel bad that your post has gotten no less than 110 views yet nobody has anything helpful to say.

The research I have done indicates that these cars just have a very poor design in this department and either need a replacement cat that is not of the original design or some other fix to keep them from repeatedly throwing the cat codes. If you search spark plug "anti-fouler" you can find a trick a lot of nefarious people use to get red of a cat altogether without setting codes that, in this case, just keeps you from being screwed by a design flaw that is not easily remedied. Further, plenty of folks report passing emissions at the tailpipe with an anti-fouler installed, so with luck you should still pass a sniffer test as well. The anti-fouler needs to be drilled larger to make space for the sensor to fit inside.

If that doesn't work, I'd try welding in an oversize cat with correct pipe diameter inlet and outlet, or at least one that has the downstream sensor bung at the far end of the cat instead of the middle, maybe also switching to one that has a ceramic substrate rather than metal as well.