Hello, error code 91 is a 'loss of communication between thermostat and equipment'
If you are handy turn off the power to air handler and furnace, and to the outside unit, then locate all of the electronic connections to the printed circuit card in the furnace, and unplug and replug each one several times, (one at time) this will burnish any oxide from the connections.
If that fails look for an burnt or corroded spots on the printed circuit card. Using a magnifying glass is helpful.
If it all looks good on the printed circuit card do this
Stress test the printed circuit card.
Use a hair drier to slowly warm the printed circuit card up to120F or a bit warmer, but not over 140F... if there is any change in the error codes or other furnace behavior... the card has a cracked micro circuit that opens and closes as the card expands and contracts... that proves a bad printed circuit card.
No change however does not prove the card is good however... but a change proves the card is definitely bad.
We can go from there.