If your low washer fluid light is on when the reservoir is full, there could be a few different causes. The most common cause of this issue is that the plastic tubing leading to and from the reservoir has become blocked or clogged with dirt and debris. This can prevent fluid from flowing through properly and make it appear as if the tank isn't full even though it actually is. Additionally, there may be an electrical problem with either the wiring to or within the dashboard gauge itself, which would also register as a false empty reading in spite of having a full tank. Finally, some cars have sensors located in their windshield wiper reservoirs that detect when levels are too low; these sensors can malfunction over time due to age or wear-and-tear resulting in inaccurate readings even when filled up correctly. If you're experiencing any of these issues then you should take your vehicle into a certified mechanic for further inspection and repair.