Got an Amana/Goodman natural gas furnace. Model CHPF1824A6AA, about 12 years old. Has worked great until late last week. LED lights blink 3 times, indicating an induction air problem. I checked the vacuum switch - infinite resistance when induction fan is off. When induction fan turns on goes to zero resistance. I think this is OK. Checked limit switch, it being cold - is closed at zero resistance (not open) - i think this is OK. I need to check the . But what else do I need to check?