If your washer fluid light is on when the reservoir is full, there are a few possible explanations. First, it could be a faulty sensor. If this is the case, the best solution is to take your car to a mechanic or dealership and have them check it out. Second, your washer pump may not be working properly. This can often be fixed by simply replacing the washer pump. Third, there may be an issue with the wiring between the reservoir and the dashboard. Again, taking your car to a professional for diagnosis and repair is the best option in this case. Finally, it's possible that you're just low on washer fluid. In this case, simply top off the reservoir and see if that fixes the problem.