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I replaced the control board, though it wasn't cheap ~$180.
The furnace has worked perfectly since.
Since the furnace would work fine sometimes, I believe a bad relay or relays on the control...

This older style HeatCraft board does not have any Diagnostic LEDs :(

Last night it actually ran fine but all the while the control board was clicking incessantly as the main blower was running....

Neutral to ground = 0
hot to ground =120V
hot to neutral = 120V

Jumpered R to W for the thermostat, but the problem was not corrected.

Inspected all the solder joints and nothing appears out...

Same thing happened again a few hours later.
Clicking noise from board as main blower starts, but flame and blowers are going full.

After a few repeated attempts, the clicking noise from the...

I cleaned the flame sensor and then re-started the furnace. At this point, the control board clicked incessantly after the thermostat kicked on and the inducer motor would not fully start, no...

We have a Armstrong Ultra Sx80 furnace, model GUJ050D10-3A, with a HeatCraft Control Board, cat number 70k98 (40403001).

Everything fires up fine… thermostat, inducer motor/fan starts, then...