Get a new tank sending unit. Both the gauge reading and the warning light are activated from there.

Diesels accumulate algae in their tanks and it often causes your problem with the sender. In a gas car it would have simply wore out by this time.

Replacement is simple. Remove the first aid kit from the rear shelf and the sensor is right there.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician