While on our trip to NJ I got a Check Engine light. Read the lean mixture error and it came out for either left or right bank oxygen (lambda) sensor. Earlier models of the R129 had less sensors, but my 1999 MY with M113 engine has two upstream and two downstream. Based on anecdotal evidence these tend to start going out around 100k mile mark so I just replaced both of them. The top ones haven’t given me trouble, but the previous owner may have replaced them at some point.