Pulling out my hair

Hey guys! I’ve got a 1996 Runner 3.4L that I’m losing my mind over. This is my daily driver and hunting/fishing rig. She’s got 340,000 miles and runs like a top. Problem is it’s time for smog. The check engine light came on and it read the old standard P0135. So I changed both O2 sensors out of habit (this certainly isn’t my first Toyota) I ordered the sensors off of Amazon. I believe they’re Kwiksen sensors. I’ve used them before and never had a problem. This time, however, the check engine light came back on. It read P0135 again. I talked to my smog guy and he thought maybe I got a bad sensor. So I bought the $100 Denso part from Napa. Changed the upstream sensor and thought I had it whooped. THE NEXT DAY!!!! P0135 AGAIN!!! So now I’m chasing wire looms and poking lead tips off of my multi meter into everything I can think of. I’ve read some other forums that said there’s a fuse?!?!? I can not find one anywhere. Has anyone had this issue? I don’t know if it makes a difference but I’m in Commiefornia. Any info would be greatly appreciated!!!!