I have two 1999 Carrier furnaces in my home. One of the two furnaces is a 58MCA120, the other is a 58MCA100 both are natural gas furnaces. The smaller unit is hardly ever used; in fact it is still like new. However, the larger unit the 58MCA120 does not heat. The flame sensor will not glow in the combustion chamber when going through self test. Nearly 3 years ago I replaced the gas valve in this particular unit. I have checked the igniter to make sure there are no problems with it and it looks good as well. I have even swapped out the igniter and the flame sensor from the other unit which works well and it still does not glow or try to fire when going through self test and the flame sensor and the igniter that I removed from this units works in the unit that is not giving me any trouble. I am getting a code 34. The filter is new and I keep the equipment clean, but now it is getting cold in North Carolina and I need heat. Regardless what is going with the unit the flame sensor should still glow in test and the igniter should try to fire up the burners in process when conducting the manual test shouldn’t it, but I am getting nothing? I need help!