It sounds like the exact cause of the "warning" light is unknown, impling it must not be labeled. If this is the case, it could be one of the retro installed warning lights for low fuel lift pump pressure. Detecting low pressure is critical because it leads to the high pressure pump failure- quite expensive to replace. A clue that the light was retro fitted would be the 12 volt power lead will not look the same under the dash as other 12 volt devices. This is the case with my 99 39Z Bounder, 275 Cummins. This retro warning light and presure sensor was added by the previous owner, a truck repair shop owner. Soon after 99 this warning light started to come as standard equipment. Hope this might help.
Wendell
99 Bounder 39Z
