It is probably one of the first things your mechanic checked, but is your alternator charging?

Your gas gauge problem is surely the sending unit. They are a not infrequent failure item.

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Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".