Had this issue for a while after pulling the coolant/washer container to do my PS and water pumps. Tested voltage all seemed good. Was ready to try replacing the sensor and was reading the repair manual for the washer sensor and noticed that the diagram showed the sensor plug was vertical or that the plug was on top. Sure enough mine was at a 45. I spun it so the plug was facing up and sure enough the dash LOW WASH went away immediately. All the searching I've done I never saw this solution.

I absolutely love when it turns out to be something so simple. I'll be the first to admit I'm not perfect. Hope this helps someone.