CEL Codes P0137 and P0455
 

I have a '99 A4 Quattro 2.8 and pulled the P0137 O2 sensor low voltage code along with a P0455 gross evap leak code. These were reset a couple of times already, the P0455 wasn't there before, but the small leak (P0442) was...

Is the P0137 usually the O2 sensor itself, or are there other things to check along the way?

P0455 is often the gas cap, it was tested at inspection and found to be good, but possibly doesn't seal in the car itself. What other likely culprits should be checked?

Thanks in advance.