When the thermostat is turned on to heat the pilot ignites and stays lit for 10-15 seconds then turns off. It continues to ignite and repeat itself and the burners will never ignite. The heat sensor (white rodgers 3049-115) was replaced with a new one and it does the same thing pilot ignites and burners don't. Tested the output of heat sensor to the gas valve with a multimeter and it does switch from 0v to 24v when the heat is active. That is about as far as I can go with this, what next?? Any help appreciated Thanks.