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Hi Rick, I'm Brian and I'm here to help! Did you notice if your furnace was displaying fault code through the peek hole on the furnace? This sounds like a limit has tripped. A code will help zero in on the issue. Thanks
Looking at the wiring diagram and reading the sequence, you could be on to something about the damper though. The damper proof switch is in the same circuit at the rollout switch and the blocked vent switch and the fan would run continuously if any of those weren't met. The damper motor is the first thing to energize on a call for heat. If the damper isn't moving then I would say either the damper blade is too tight for the damper motor to overcome or the damper motor itself has failed. If you can open the damper manually and the furnace starts, then all the other safeties in the circuit are good and it's probably the damper motor itself. The damper motor is energized through the control board so there's a possibility there is an issue with the relay that energizes the damper too. Testing for voltage at the damper motor or across the damper motor relay contacts is the only way to determine for sure what's going on. Let me know if you have a meter and would like help testing.