I have a Niagara forced air gas furnace. No idea how old it is, but it's a big green box. House was built in 1958. Recently it stopped working properly. It would not turn on automatically through the thermostat controls. I thought the thermostat might have died, so replaced it. No change. I am posting a picture of the controls area for reference. It has a pilot that is always on. The main valve has a thumbscrew on the top that allows me to open and close the valve manually. When I open the valve, the furnace operates as it should - main burner lights, jacket heats up, blower turns on when jacket reaches correct temperature. Shutdown when I close the valve also works properly. With all this, it seems to me that whatever controls the communication between the thermostat and the main valve is where the problem is. I just don't know which part that is, or how to test it. Any help would be appreciated.

Greetings all, Each year I clean the flame sensor with sandpaper before winter comes. I've never run into an issue until this year. After cleaning the sensor and re-installing it, the furnace will not start. Looking at the control board, the status light continually flashes a fast red. I have done the basic reset of lowering the temp on my thermostat, unplugging the unit, waiting for awhile, replugging it in, and then raising the the temp on the thermostat. I still receive the same error and nothing happens. The connection to the flame sensor seems good. I didn't monkey around with anything else in there. Did I touch the sensor to something to cause this problem? Any ideas? Thanks, Brad