Hi and welcome to the forums. It could be debris or some kind of build-up messing with the sensors, or a control board could be bad. First thing I'd try is cheap and easy: blast out the bad area with canned air. Really get the straw in there and spray all around the affected area. Also, turn the keyboard upside-down when you do this so gravity works for you, not against you. This *may* work, but my guess is it needs to be opened up and maybe some parts to be fixed.
So if the air doesn't work, you may want to look at just getting a new one. 9 years is a pretty good run for an entry-level digital piano and you will be getting a MUCH better sound and feel with a brand new instrument. Brand new CDP-135 is $299. So food for thought.. GOOD LUCK! Let us know how it works out!