[kaisen]

We'd certainly need to know more to be of real help.

A lean code at the front sensor could be a sensor error of course. You could check the voltage and see, or just swap another one. Or, they may have set the timing slightly off when they did the belt. Or, you really could be running lean -- clogged injector(s), MAF sensor thinking it was getting less air than it really was, faulty knock sensor dialing back timing, vacuum leak, carbon build up on a valve where it doesn't seat 100%, lots, and lots of little possibilities.

The rear sensor, bank two, is the real policeman of catalyst performance and it isn't throwing a code (right?) so the cat is doing its job and, generally, AFR is at Lambda.

Toyotas are funny. Some code readers won't get the whole story. I'd suggest letting a good Toyota technician have at it. It's likely something really simple. But, it still needs to be repaired so you don't do any damage.

How are YOU resetting the codes?