2003 3.4 P0420 Post intake coolant leak

Before driving from CT to UT, I finally fixed my coolant leak due to bad intake gaskets. I had been leaking progressively worse for months, and burning coolant at a pretty alarming rate until I fixed it in January. I subsequently replaced the spark plugs and changed the oil of course. Everything went fine and the drive was smooth, 35mpg across the country. Just last week she decided to throw a P0420. Drivability is fine, though mileage seems to be down to around 31mpg on the highway. The exhaust is very new all around, including the cat.

Could the burning of coolant have fouled the sensor(s)? I monitored the voltage while idling. Sensor 1 fluctuated up and down from around .1 to .8 mV. Sensor 2 stayed relatively steady between .3 and .5 mV. RPMs weren't as steady as they used to be, but not bad. I'm highly doubting a leak due to how new the exhaust system is, does it make sense that burning coolant for so long has fouled something? And if so, how do I check? What will a coolant fouled O2 sensor look like?

If it's relevant at all, I also had to replace my water pump after I got here. I really did a number on the coolant system running it low so many times. If only my coolant level sensor worked...

That second O2 sensor should be going low and stay there and then go high, and stay, slowly changing high to low and back......it should not be staying near the middle.....either a bad sensor(contaminated by coolant?) or bad cat....

I've been driving around with my scanner hooked up and watching sensor 2 under different conditions. I thought maybe it wasn't heated enough the first time around. But it still isn't acting like you say. It idles right around .450, and seems pretty numb to change unless I really stomp on the throttle and suddenly close it.

So what does burning coolant for so long mess with? The cat? The sensor? Both? I'm too poor for a new cat so I'm hoping it's just the sensor, but how will I know? I take it it's somewhere behind the cat? I've never noticed it in my time under the car.

Also, if it is the sensor, why the second and not the first?

Well, you said the cat is fairly new, so my money would be on the rear O2 sensor.....you said a new exhaust is in there, just check to make sure that the harness to the rear O2 sensor is routed correctly and there is no short or open in it(burned or cut wire)....

The front O2 sensor is for fuel management and won't set a P0420.....the rear O2 sensor tests catalyst efficiency, and will set a P0420, if it is contaminated......bad cat will do the same.....my money is on the sensor.....

Ok I'll take a look at the sensor. From what I've gathered I'm looking for a whitish coating on it. I didn't do the exhaust, but I trust my shop. I was wondering why the second would get contaminated and not the first IF it is because of the coolant, which has been the root of every issue I've had with this car in the past year. Or if burning coolant destroys cats, which I'd assume it does.

Edit: Went up to the mountain today...slowly. The cat is definitely clogged, only noticed it trying to climb hills. My worst fear, since it's the most expensive. Here's what I was wondering all along, ripped from some other forum:

3. Oil or Antifreeze Entering Exhaust

When oil or antifreeze enters the exhaust system and contacts the hot core of the converter the oil and antifreeze will burn off leaving carbon deposits. The carbon deposits will coat the core of the converter thus reducing the catalytic converter's ability to convert from harmful emissions into harmless compounds. As the carbon deposits continue to accumulate, the pores in the ceramic catalyst will become restricted and block exhaust flow through the exhaust system. The resulting increased backpressure will result is a loss of power and overheated engine components. Possible causes are worn piston rings, faulty valve seals or valve guides, blown head gasket or intake gaskets, or warped engine components.

So yeah, now to find cheap/creative solutions to this problem. Good thing my emissions is pretty recent...

Ok so it's obvious to just replace the cat. 2 questions:
1. Will any aftermarket brand do? Any to avoid/prefer?
2. Is it as simple as it seems? Just unbolt and bolt on? I'm quite broke and thank goodness I brought my tools with me, but not my jack so I can't get under the car right now to case the job. I'd really like to do this myself, as with all work on my baby. She's taken me 245k+ miles all over the country with no problems I didn't cause myself, she deserves to be treated well.

Just to update in case anyone ever reads this. So I put in the bolt on Magnaflow replacement and the problem is solved. Mostly. I think I may have done some permanent damage to the engine, because it seems to have lost a lot of power. There is also occasionaly a...putt putt sort of sound from one of the cylinders that beats in time with the engine. My guess is a burnt valve or gasket, but it's not always present so I'll leave it for now. Granted by the end I had learned to drive the car so lightly the code disappeared, trying twice to drive it up a mountain before I knew what was wrong (well the first time, the second time I knew what I was doing and did it anyway) surely did some damage.