Ronspool

I have a 15 year old Jandy Lite 2 that has been working fine, but just in the last few days it isn't keeping the pool up to temp. It is showing no error codes, and is heating, but can only get the pool to 83 degrees while it is set at 88 degrees. The display shows the heat at 83 degrees, so it doesn't think the water is already at 88. Pumps are working fine. Any advice on how to best trouble-shoot?