Furnace: 4 flashes => open high limit

I have a Goodman gas furnace (GMH950453BXAE) that blows air but gas does not fire up. The circuit board light shows 4 flashes, which is open high limit. I’ve checked continuity of both high limit and aux relay and they both check good. I’ve also checked for air obstructions both into and out of furnace and I do not find any. Any ideas of what else to check? I’m hoping I do not have to have the circuit board replaced.
Thanks for any help on this!