OK, that means that the bulb and socket and the wiring (printed circuit and discrete) and such is good. The problem is therefore in the sender or the wiring between the sender and the instrument cluster. My guess would be dirty float contacts.

The light is powered in the "test" position (key ON, engine not yet started) by the ALTernator light. That's why it goes off once the car starts.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970