Problem code P0420,P0430

we have a'96 Ford Windstar GL, it's got 219Kmiles on it, check engine light came on, pulled the code got P0420 Catalyst Efficiency below threshold (Bank 1) and P0430 (bank 2). Can anybody possibly tell us what this means? What possibly caused this problem? What's the cure?
Also I would like to add about 2 mo. ago being we where having a misfire problem which was solved due to clogged exhaust ports. We took the van into a muffler shop to have the catalytic converter checked which it passed. Said there's nothing wrong with it. In order to perform their tests, they drilled holes into it to put test probes into it.I'm wondering if due to their process this could have anything to do with the problem?
Appreciate any feedback. thanx.

They drilled holes into the catalytic converters to do a test???

That soinds like what we call in the enginering world "a destructive test".

I am no expert on catalytic converters and the testing of them but drilling holes into them doesn't quite sound right.

I would have thought that they would have simply measured the efficiency via exhaust gases.

I thought it didn't sound right either and now that we reflect on it a little bit we jumped the gun on the whole deal because we were told in the beginning if the engine misfires continually (which it did)this could cause the catalytic converter to fail. So I call the shop he says yeah we test them for free bring it in. At the time we were having strong exhaust smells like too much gas. So I took it in there told them I could only wait 50 min. then I had to go get my son off the bus, he says no prob we'll be done in 20 min tops. They get the van in there, I wait 30 min. no answer i went up to counter and the guy was pissed off that I had to go in 10 min.and his test ports were still inthere and it might run crappy but don't worry we'll fix it-yeah destruct it. I brought it back 5 min later& they took the probes out and welded something back?Now P0420 codes about 2 mo. later (even after solving the misfire problem due to clogged EGR ports and replaced upper intake manifold gasket,tune-up)and there's a real strong exhaust odor in the cabin and really not good for us I imagine so we gotta do something. The same place wanted $160 for a catalytic conv.installed but was mad because there was nothing wrong that they could find with it and I wasted his "man hours for nothing" and he could charge me $25 bucks for the time it took? I won't be back but would like any advice you all have to offer? thanx

Exhaust into the car is very dangerous and unhealthy.

The CHEAPEST catalytic converter that I have found is about $300 from www.rockauto.com
It is a Walker "direct fit" that is both catalytic converters, and the pipes to the exhaust manifold, and to the flex pipe in front of the muffler.

List price for the Genuine FORD part is about $1600.
I found the part online for just over $1200.
I bought and installed the Walker....for the price....I thought it worth a try. It has been on for almost 1 year.

It was NOT a fun job to replace.
One other person bought the part....and had a local shop install it for them. They were charged $100 for the installation. Working with exhaust is a real pain.

You have to be VERY carefull with the exhaust manifold studs.....I ended up replacing 1 of them.

You also MUST be very careful with the oxygen sensor that is mounted into the rear of the rear-most catalytic converter. When I applied very gentle turning ONLY pressure to the sensor....the threaded part on the converter came right out......which would have been a major pain....if I was not changing the converter out anyhow.

IF you should end up installing new oxygen sensors.....I would replace them in pairs......as in both "upstream" (before the catalytic converter) and / or both "downstream" (after the converter).
This will give you the same switching charatoristics on both sides of the engine. I also would buy the Motorcraft brand. I bought mine at Rockauto.

Check the condition of the flex pipe that is connected to the rear of the converter pipe....to make sure that it is not leaking. You can also get it at Rockauto.

I have pictures of the original part and the Walker replacement part in my pictures at http://community.webshots.com/album/201931518cScpNK