On my 2001 E320, I just replaced the MAF after OBD codes indicated PO173, etc. I learned from the dealership that as the component begins to fail, codes are stored. The check engine light will not be triggered until the problem exceeds computer standards (maybe your MAF was just beginning to fail).

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1982 300D-T (sold)
1988 300E (sold)
1994 E320 Wagon (sold)
1995 E320
1998 E300D-T
1999 E3204M Wagon
2001 E320
2005 E320 CDI