My aunt has a carrier comfort 80 in her house which stopped working this past winter after operating fine for several years. I has a error code 33 and the fan keeps running, but the heat will not come on and the fan will not stop even if the thermastat is set for auto and off. The only way to stop the fan is to unplug it. They have replaced the therma stat and filter, but the problem has not changed. Any ideas on what I could do to correct the problem?