I have a Whitfield profile 30 FS from the early 2000s. I’m having trouble with the pellets turning black and not burning all the way and building up in the fire box after about 2-3 hours. I replaced the door gasket, auger motor, control panel, exhaust fan, combustion fan, cleaned it top and bottom. I read about recalibration on the control panel but, It seem to do nothing different with all the changes. I’m losing the battle and about to give up any ideas what I could do or tweak would be great thanks!!