Lets start with safety. Check your vent pipe for obstructions, mainly outside at the vent cap. The safety of this unit will not allow it to operate if there is anything blocking the vent. If all is OK, then next we will light and observe the pilot with it on the pilot setting on gas valve. The pilot flame should consume the tip of the generator point by 1/2 to 5/8". If it does not, then screw off the pilot adjust screw cover on the gas valve. This is clearly marked on the top of the valve. Inside you will have a much smaller screw. Turn it counter clockwise to raise the pilot level, clockwise to decrease. Never consume more than 5/8" of the generator tip. Hopefully your problem will be resolved here at this step. If however it is OK, then take a millivolt meter and read the generator millivolts, at the gas valve with the thermostat contacts closed. It should read 140 millivolts. You need this much power to properly operate the heater. If it is lower than 140, then replace the pilot generator....Rob Just a note...Do not remove or take apart any part of the Gas valve, other than the pilot adjust screw cover...Good Luck.