Can you elaborate? I interpreted the same as the other comment that the blower fan just needed a hand start and kept going after the furnace started up. What you're saying only makes sense to me if the spinning the fan by hand allowed the furnace to start by bypassing the safety at startup, but wouldn't that mean that if the exhaust fan was stopped during normal operation (blockage etc) that the furnace would just keep going, dumping CO into the home?

I was spinning it in reverse actually, but it would be enough to start the exhaust blower. It would also re-start pretty well for ~6 hours. It was probably the bearing. Also FWIW I have multiple carbon monoxide/air quality monitors and nothing tripped or alarmed.

From the description I thought that a degraded capacitor or lack of lubrication made the blower not start on its own, but the blower (and the whole furnace) would work if given a manual startup spin by hand.

Welp, AI almost killing someone is definitely an "oh shit" moment.

https://www.covenantairesolutions.com/post/what-is-a-furnace...

“At its core, it's a small motor with a fan attached that has one primary job: to vent harmful exhaust gases out of your home before the burners ever kick on. This is the very first step in the heating sequence, and it's non-negotiable for a safe startup.“