Installed cnt 03797 board now I get code 3 pressure switch problem instructions call for disconnect pressure wires what could be problem bad board

Restricted vent or vent motor not working. Vent pressure switch stuck open or closed.

Vent motor is working pressure switch is not needed with new cnt 03797 control instruction called for pressure switch be disconnected

I can kind of see what your saying. Not knowing make and model of the furnace only slows our process. If your system is inducer draft(vent motor) It will have a pressure switch, vacuum switch or centrifugal switch. One of the above will have to close a circuit to prove the motor is running and the vent is clear for it to light.