Fires up for 5 to 10 sec only

photo cell is dirty its alittle sensor thats senses a light of the flame in the combustion chamber.if you take the top cover off and look to where you see a thing that looks like a spark plug right under it or around that area you should see a senser with two wires coming out of it pull it out the bracket that holds it in place and clean the lens with a cloth then put it back and fire it up it should be find if not then the sensor is faulty