What do you mean by 1st low warning light? I ask because the gages in a semi are normally set up as 4 green blocks (1/4,1/2,3/4,full) The 1/4 light turns yellow as a warning then red before the engine goes into protection mode. They are not the most accurate gages and they do fail. Had to have one replaced a month ago on a 1 year old truck. So it's possible you have a bad gage or a leak since you have added so much DEF. I'd start with looking for a leak. DEF leaves a white crystal residue that's caustic. If you have a leak you need to find it and protect your truck.
