I too went to AutoZone and had the codes pulled for a CEL. I don't recall them right now but they were something related to left bank and right bank (I may have even posted them in a plea for help on one or more of the ML enthusiast sites)

AZ said O2 sensors and said which ones (there are four (4) I believe). AZ wasn't sure but they were eager to sell me either exact replacement or universal replacement "to save me some money".

Read everything I could find on the WEB and decided it probably was not the 02 sensors. Replaced the MAF with a screwdriver and 10 minutes.

CEL went out. Engine performing flawlessly...that was 11,000 miles ago.

Hope this helps.

__________________
Jim