My heater kicks on for about 15 seconds then turns off and the fan runs indefinitely, after a while the heater will kick on again and the same thing repeats. Meanwhile cool air from the fan is blowing in the house and the temp on the thermostate gets lower and lower. I've tried turning off the emergency switch on the wall for about 30 seconds then turning it back on hoping the heater will reset itself but the same thing as above repeats. Could it be the gas valve and where can I find out how to fix it? Thanks inadvance.:)

Look on the circuit board and see how many blinks it is doing. This is a diagnostic tool. Post results.