Without knowing your furnace, there are things like the pressure switch on an exhause booster fan, proof-of-flame circuits, etc.
Things like this are really a job for a properly skilled technician. You do not want to try to test or repair the gas control valve. Generally, there are probably multiple safety circuits involved, and you need a detailed wiring diagram of the unit to work on it. The wiring diagram may be on the inside of the cover.