First time preventative maintenance

Yes, you are wrong. It can be either sensor, or both, but it's typically not a problem with the cat, unless you ignore a rich/lean condition for too long. From the megathread:

• Cat inefficiency codes P0420 and P0430: Strong chance it's not your cats. Read why HERE.

 

Well I figured it wasn't the cat. I just wish the obd2 scanner was able to tell which sensor it was, instead of just saying 'bank 1 sensor 1'. I'm wondering if the truck's age has something to do with the lack of quality reading on this.

 

You replace them in pairs generally anyway.

 

What @KNABORES said.

And there is a way to tell if the sensors are reporting out of range. That info is in the FSM. But I’d just replace both on the bank and be done.

Generally speaking, and I do mean GENERALLY, if you’re not seeing unusual performance problems - knowing the upstream are responsible for helping the ECU determine fuel/air ratio, and adjust all that stuff - it’s probably not the upstream. Downstream is usually half the cost of upstream. So if someone was in a pinch, I may tell them to swap the downstream first. However, we’ve definitely seen cases where it was the upstream, there were zero running issues (rich or lean or stumbling or weird idle), and you really should replace in pairs when possible.