As I come to a stop (or reverse away from one) I get a light rattle from underneath. Searching here for similar complaints it seemed the most likely cause was something in the catalytic converter so naturally I started getting whiffs of rotten eggs and loss of power on acceleration.

Well, here is is a week later and the SES came on, code P0420 (catC). I am definitely experiencing a loss of power on acceleration. And still the rattle.

Possibly related - a few days ago I lost one and a half gallons from the radiator due to a loose cap and before that I got a code for MAF, which I cleaned and the code is now gone. Could be one of those events changed up the air/fuel chemistry and messed up the cat? If so, there's not much point in replacing the cat because whatever the issue is, it will mess up a new one also.

The only code now is P0420 (pending). Ideas for what to check to see what is giving me a power loss?

 

Would a bad catalytic converter cause a loss of power? I can barely make 50 mph and when I do, the exhaust note is more like a go-cart than a 6 cylinder blazer. I'm thinking that the power loss is caused by something else, and that something else is causing a catalytic converter P0420 code. So I am reluctant to blame the Cat when the problem could be something else. Unfortunately I erased the CEL and now am just getting Not Ready codes.
Car has 99K miles. I guess it could be the cat, there is the rattling and the sound. Hoping there is another, cheaper, cause. Could there be?

 

Replaced the catalytic converter and all my power came back, so that was the problem. Honeycomb inside collapsed.

I had a local shop do the work for an estimated $300 labor to install universal cat. However, on my 2door 2WD Extreme, they couldn't weld it on without removing the crossmember and even then had to remove the entire assembly up to the manifolds, weld it on, then reinstall it. So for those thinking about going the cheap route, get under there and check out how you are going to cut and weld just the cat. You may decide it is better to pay a little extra and buy the whole assembly, all the way to the manifolds.

After a week my check engine lit up, gave me a bad MAF and O2 sensor behind the cat. So I think the shop just erased the codes and they came back. I waited and another week of driving and they disappeared for good.
And as a PS, when they reassembled the exhaust, it was turned slightly so that at exhaust tip the tailpipe was rubbing against the bottom of the quarterpanel. Made a buzzing sound. So I cut away one inch around the rub spot, and all is well.