My wife`s es300 started loosing power and chugging and the check engine light came on, luckly she was close to our mechanic and he is looking at it. the scan says its running lean, I know it needed new spark plugs and he checked them and they were worn out, so he is going to change them then look at the o2 sensors. Any other ideas that could cause the lean problem, and why did it happen at once, running just fine one minute and bad the next?

Running lean, I would look at the O2 sensors as those are the ones that send air/fuel mixture signals to the ECU. The plugs just create proper spark/combustion.

FIXED-wife is happy again, took longer to fix because when the shop went to install the 02 sensors they had the wrong ones, turns out my Lexus has California emissions, and I live in Indiana. they figured the o2s fouled the spark plugs or the plugs fouled the o2s.