My SSEi is still running fine but want to be proactive. Don't have symptoms yet..just codes. Car has 300k miles, original factory exhaust. Don't have money at this time for a new cat. Would a temporary pipe replacement cause me troubles? Thanks for your input...

What codes is it throwing?

P0420
P0436

A 2003 SSEi can use a generic CAT just fine, had mine installed parts and labor for under $150.

Agree, a generic from the average muffler shop would work fine for quite a while if that'* within reach.

The title says "clogged" and that to me means the guts of the catalytic converter melted into a ball of guts at the outlet side of the catalytic converter. When this happens, it is usually caused by post-combustion-chamber combustion. Think misfires or no spark or etc. that causes fuel to NOT get burned until it meets the core of the catalytic converter, where it burns a lot hotter than the Catalytic converter is designed for. I've had this happen on an LT1 of mine, an LS1 of mine, an Oldsmobile 307 of mine, and with several friends' cars. On the LT1 I didn't want to be stranded so I got out an angle grinder and went to town cutting slots in it until it drove okay enough (plus noise).

This part makes me think you don't have a plugged catalytic converter, but rather one that is worn out therefore doesn't get warm enough to do its job no matter what the PCM tries to get it to light.

These codes appear to confirm my thinking.

If that is accurate, you won't see issues for a long time other than the smell from the tailpipe, the light on the dash, and slightly-lower fuel economy. In this case the catalyst is all worn out, probably starting to get coated by carbon like the rest of the exhaust system, and isn't processing the exhaust properly. In this case it won't melt and will probably not clog up for tens of thousands of miles or more. Only real problem I'd predict is that when you do finally get it replaced you'll have to replace both oxygen sensors at the same time. They'll be all carboned up and do a bad job of measuring the new catalytic converter and likely ruin it. Bright side is you probably needed new oxygen sensors anyways and they're not horribly expensive.

The other potential issue you may run into is a chunk of the core of the catalytic converter breaking off inside and turning sideways at the outlet end of the catalytic converter. This will plug things up and make the car run poorly or not at all. Keep an ear out for rattling from the catalytic converter, that'* your only warning at this stage.

If you are in an emissions-controlled area and have a deadline to replace the catalytic converter, plan on doing it about a week before your test. Replace the catalytic converter and all oxygen sensors at the same time then drive it around so the PCM has time to figure things out before being put on the spot.

. . . then I replaced both of them the following weekend.

Do they use exhaust "sniffers" for the emissions testing where you live or just plug into the OBD2 port to get the readings off the PCM like here in NY?