Could be a number of things. You should be able to get a readout on the display to tell you the problem. However, in the past the usual problem was water hit the controller at the junction block for a remote temperature sensor. The water completed the circuit and then it thought a remote sensor was installed. When it dried out it then gave an error that the sensor is missing.
First thing I would do is: follow this sequence --> Remove fuses from battery leads from controller to batteries. Remove hot wire from solar panel from controller. Be carful the wire has power. Do not touch anything with it. Wait a minute. Reinstall wire. Then reinsert fuses. This is how you reset controller and it should clear the error. If the problem was the temperature sensor you can jump the two terminals with a small piece of wire and it will always think the remote sensor is installed. I also relocated my controller so that it avoided water. I also installed a fuse in the positive leg going to the controller so reset only requires removing the fuse. As an aside, you may want to put some corrosion block on the controller terminal screws to avoid future corrosion. Once corrosion forms you may not be able to loosen the screws later on.
There are other problems that can cause the waning light so read your directions on how to read the codes.
