2001 VW Beetle P0131 O2 sensor PLEASE HELP!!

I replaced the upstream O2 sensor due to the code (can't remember the #) but said there was no activity O2 sensor bank 1. Car now runs great, but the CEL came back on and the code is now P0131 low voltage O2 sensor bank 1. It is the 4 cylinder 2.0 I think...If you have any specific ideas what this is caused by, please post answers for me!! THANKS

Well I found on some other forum that sometimes just a tune up will take care of this even though it runs fine and there are no misfire codes. So I put in new three electrode plugs and premium wire set, reset light, and it has stayed out since a couple of days past when I first did the posting.....This appears to have fixed it.