Thanks for the help guys, I finally changed both sensors. I got them for $113 rear and $119 front and the exchange took only a couple of minutes.
Can I reset the check engine light or do I need to go to a shop to have it reset?
cheers,
Sab.

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regards,
Sab.
'07 ML320 CDI (398 ft lbs / 1400 RPM)
'02CL 500
'91 928 GT Supercharged