At 180,000 miles and a P0420 code to troubleshoot, I figure I may as well replace my oxygen sensors and rule out that possibility.

I found a name-brand downstream O2 sensor for cheap, but the cheapest upstream ones are all universals. Should I drop the extra bucks for a OE-fit sensor up front?

Yes, well worth the few extra bucks.

Universal fit means cutting and soldering. Rock auto has a denso for under 70 IIRC

i've used the universals on man cars i have worked on all i do is cut the plug off the old one and wire the new one to it. very simple

Oh yeah you don't need a down stream. That'* only there to check to see if your cat is working. Also I wouldn't chase any codes that work off a sensor till you fix your battery cables. So you are not chasing a phantom code.

I've used a few Denso from rockauto, good price and they last a while.

I think the last couple denso'* I got from Rockauto, Dan had metnioned they were under $40.

Get Bosch and expect to replace it soon after. They don't seem to get along with our cars.

Do you have a scan tool that you can see your cars data? I would scan the data before I would spend money on a sensor.

If the universal-fit sensors require soldering, I'll gladly spend the extra money on a sensor with the proper connector. I've already heard plenty of Bosch horror stories.

Last time I had a scan tool hooked up, the O2 sensors appeared to be working; the rear graph looked mostly like a scaled-down version of the front one, which suggests to my untrained eye that the cat is still working, at least at idle. But this was before P0420 codes started popping up.

Naturally, this is definitely something to be handled after battery cables. Just brainstorming out loud, that'* all.

Also make sure the plug wires are not touching the O2 sensor wire. I was scanning a car that had real odd O2 readings. I scratched my head a bit and then popped the hood. Yep, a plug wire was laying right on the sensor wire. Moved things around and the readings returned to close to normal.