From what you have described, it sounds like your heater has not been cleaned and serviced in quite a while. Further, the model number you listed is for an LP Gas blue flame convection heater, not an infrared heater like you described ("2 of the panel turned black and will not burn".
Which ever it is the solution will be the same ... The unit and the gas to the unit needs to be turned OFF and the front of the heater cabinet needs be removed, so you will have access to cleaning the interior and the mechanical parts. You will find sheet metal screws on each side of the unit and the top, that will have to be remove to remove the front of the heater. Using a shop vac with a hose and brush attachment, start cleaning the interior of the cabinet. Then move on to the radiant plaques if so equipped. Vacuum them carefully and the remove them and set aside, where you won't accidentally step on them. Now, you have access to the burner and the pilot assembly. Vacuum the burner thoroughly and also vacuum the pilot assembly. Switch to a can of compressed air with a straw nozzle (like you clean your computer with) blow out the pilot and the ODS Sensor (Please see link to your Owner's Manual,on how to properly accomplish this). After everything is thoroughly cleaned, reassemble the heater in the reverse of how you disassembled and remove the plaques. Turn the Gas ON, purge the air from the gas line by holding down the gas control knob, while in the PILOT position. Once you smell gas, release the control knob and wait 5 minutes before attempting to light the pilot.
To help you further, I have included a link to the Owner's Manual for your model DESA Vent-less heater model you listed. You can download and print it for future reference. http://www.desatech.com/manuals/Vent-Free%20Heaters/107884-01.pdf
I hope this helped you to solve the problem and provide Annual Maintenance to the unit. Thanks for choosing FixYa.