I have a Portfolio light dimmer control unit with the receiver under the canopy of my ceiling chandler light. The sending unit is remote. It worked well for over a year, but then the light ceased to turn off with the remote sending unit. I changed the battery, and the dip switches are the same for both sending and receiving units. I do not know which is malfunctioning: the sending or receiving unit. The receiving unit is low profile and is in the shape of a small horseshoe so that it can fit comfortably inside the canopy of my light. Any suggestions for repair?

I believe you will need to replace the entire assembly both in the fixture and the remote. With out spare transmitter and receiver you cannot test to see which item is defective unless the manufacturer has a mail in repair program and if that is the case you can ship both items back to them for testing or replacement.
I am sure you did not want to hear this but that is the way it is.

I would also check for any loose wiring while you have the fixture apart. You might get lucky and find a loose wire.