Exhaust help

[orbot]

96 3.4 auto 4wd 240,000 miles.
ive had the truck for a year now, fixing things here and there, recently i have tha p0420 code so i start checking trying to figure out if i should just replace the o2 sensor or the cat, did the code reader test, S1B1 tests good as far as i can see, now the other sensor is kinda off so i guessed to replaced the rear o2 sensor, i crawled under the truck to make sure and i have both sensors BEFORE the cat....... isnt one suppose to be after the cat, im now thinking previous owner messed with that, now i need advice on getting everything in the right place.

Are our trucks suppose to have 2 sensors upstream and none downstream like mine has.
or
some dumb fu.. placed the cat in the wrong place, i do see someone replaced the tubing at a certain time. so its
o2 sensor - cat - 02 sensor??? 96 dont use a a/f sensor so say other members.

 

[freshley]

Only one of the sensors is actually an o2 sensor. The other is air/fuel ratio sensor. I'm not sure on this but normally the pre 2001 models only had one cat unless it is a cali emissions then it also had two. All years had 2 sensors but all different part numbers for some reason or another.
Mine is a 2001 with 2 cats and if I remember correctly, one is before the first cat and then next is between the cats.

 

[TheDurk]

Pre-1999 all 3G 4Runners had 2 O2 sensors and one cat. The sensors are upstream and downstream of the cat.

A/F sensors were only on the two-cat system, which were on '99 and '00 Cali-emissions 4Runners and all '01 and '02 4Runners. No A/F sensors in 1996.