Fuel Lever Sending Units
To test the Sending units You Will have to drain The Tank and Remove the Sending Units (Both Sides) Then test With a Ohm Meter..You can Test the Combi Gauge with the Star..Or HHT
Chances are the Sending Units are Defective...
I Have Replaced Several of them....
When I Worked For Mercedes
You Might Want to see if the Dealer will GOODWILL Them
as its a KNOWN Problem!