If your 2008 OM642 is the same as our 2012 model, your MAF sensors are located in the "batwing" intake assembly that connects to the Turbo and the air filters. I don't believe you can buy just the MAF sensors, they are a part of the batwing itself and you have to buy the unit as an assembly. At least it is that way on the 2012. It will come with two new MAF sensors already installed.

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2012 Mercedes ML350 Bluetec 102K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 238K (mine)
1998 Volvo S70 T5 Turbo 211K (kids)
1994 Ford F150 4WD 246K (firewood hauler)
1983 Mercedes 300D 384K (diesel commuter)