Help with code PO134

I have Mercedes Benz 430s 2002 . The ,, check engine ,, light com on ,i have one of those code rider and is showing code ,,po134,, and ,,po135,, .Chang one of the oxygen sensors on the driver sayt,it was looking bad ,reset the light and after a day the light com on ageyn !! Checked with the code rider-theseym code!!Eny one what is this and how to fix ???THX