blue sender is nothing to do with gauge, thats for the ecu. its the black sensor which is for the gauge. if car is pre-90 there is a 42 or 43 relay above the fusebox which is for the level sensor warning system, pull this out and it will disable it. the issue could also be the temp sensor, somehow my mates one failed in such a way the dash would read fine but the light was triggered. try bridging the pins on the header tank level sensor with a bit of wire, if that sorts it the sensor is bust. if its a pre-90 you can remove the sensor to clean the pins, if post-90 whole tank needs changing