Exhaust fan comes on for two seconds and goes off. Control board light is steady no blinks

If you don't have a fault code on the control board when it shuts down.I would say the control board is bad. You can try jumping the wires R to W at the thermostat to make sure you don't have a bad thermostat or heat anticipator. If it works the thermostat is bad.

Thanks. I took off hose to pressure switch. Not clogged. Reinstalled. Furnace working. Could thermostat or pressure switch work intermittently?