Tuesday morning as I was leaving home for work my Runner was not running right. On acceleration with a cold motor the acceleration felt really bogged down and pulling up to a stop sign was idling rough. Once the motor warmed up it seemed to be running better but still felt sluggish on acceleration.
After work I had the same symptoms and about 5 mins after first starting the truck and driving the CEL came on. When I got home I scanned for codes with Torque Pro and PO133 popped up. O2 Sensor circuit slow response Bank 1 Sensor 1. Ordered a new Denso sensor Part # 234-4162 and planned on replacing it this morning.
I crawled under the truck last night to treat the nuts with penetrating spray to discover both nuts were rusted badly and looked to be literally disintegrated to the point I'd probably need a torch and other specialty tools I don't own to change it and decided I'd call around to find someone to do it for me this morning to avoid damaging my Cat trying to remove the nuts.The studs themselves looked fine.
I called Monroe Muffler here in Lockport NY assuming they deal with this all the time and would be able to easily take care of it for me. The guy I talked to on the phone said that they would most likely break and I'd need a new Cat if they did and they would not be responsible for any damage... which sounded to me like we ARE going to break them to sell you a new Cat and screw you on labor as well.
I called a local multi generation family owned shop and he said bring it in. Less than an hour later I was driving away and my truck is running better now than in this first year I've owned it.
The peace of mind was worth every penny of the $68 dollars they charged me.

You did everything right.

“The studs themselves looked fine.”

At this point you were channel locks or vise grips away from doing this yourself.
I’ve done this on two 2000s and nuts were beyond a wrench and they broke free with little effort after the soak.

Calling BS on that first shop was brilliant.