I don't have the manual for my gooman furnace,it was installed in 2003. The glow plug comes on but the burner won't light it does this 3 times and locks out

Is the drain trap on the side of the furnace full of water? If not, dump some water in it and try again. Let me know if this works. Also, check to make sure your vent outside is not obstructed in any way. It sounds like one of the pressure switches is closing wich allows the glow plug to glow, but the other is not closing wich will cause the gas valve to not open. Let me know what you find and we will take the next step. Oh, and make sure the gas cock and gas valve are both on, and if you are on propane, make sure you have propane in the tank.