Pretty sure a 126 unit won't work in a 123 unit. But I think a 123 gas unit will work in a 123 diesel just as a 126 diesel and gas unit is interchangeable.

Let me rephrase that, it will probably work, it just won't read empty when empty or it will read empty when there is still fuel in there.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD