Performed a heating pm on a gas furnace. Cleaned the thermocouple, checked the gas pressures. Replaced the air filter. 16x25x1 Started and checked the safeties. Performed a combustion test. System is operating properly. While cleaning the thermocouple and relighting the pilot, the gas valve was not recognizing the pilot. Attempted to use 2 different thermocouples and holding the manual pilot button for up to 9 minutes and the gas valve did not see the pilot. The gas valve is bad. Picked up and replaced the gas valve. Started and tested. The system is operating properly.