About a week ago, our furnace, a Carrier 58rav115-20 stopped heating. My wife had to turn off the power switch, to keep the induction fan from running 24/7, even with the thermostat "OFF".
When I had a chance to look at it, I found that after being powered down for several days:
1) The induction fan comes on with the power, whether the T-stat is set ON (requesting heat, or not), or OFF.
2) Error code 21 flashes for as long as you let the furnace run (hours, if you let it).
3) Error code 21 flashes, with the T-stat R wire disconnected.
4) After the initial relay 'click' at power-on, there are no other relay clicks (no attempts to enter the ignition sequence).
5) At power-on, there is immediately 2.4VAC (two-point-four, not twenty-four) to the gas valve (there are 24VAC at the transformer secondary).
6) The gas valve blue wire is not shorted to COM, or GND (I see ~0.87 MOhm)
The control board is an 'ancient' HK42FZ009 1012-940-L (which I understand has a history of problems). I bought a used board and the symptom is the same. So, I'm wondering, is that a coincidence? Or, is the problem not in these two boards? Should I keep trying boards, or look elsewhere?