Not sure I am understanding the question correctly due to mistakes you made in your question HOWEVER I believe you meant that only one burner is igniting??
If this is the case...just know the customer should not be using the furnace until fixed as this is a very dangerous condition.
1)Using a manometer check your pressures before the regulator (if one is present), at the inlet of the gas valve, and the outlet of the gas valve.
2)Natural gas you want about 7-12" WC on the inlet (11-14" for LP)
Natural gas you want 3.5" WC on the outlet or manifold (9.5-10.5 for LP)
3)Make sure the vent for the gas regulator, if applicable, is clear of obstructions.
4)Make sure there is nothing obstructing manifold/burner orfices.
5)Make sure draft is correct and/or the draft inducer is functioning properly
6)Make sure no cracks are present in heat exchanger
7)If there is a flame carry-over rail, make sure it is intact properly.
8)If supply gas pressure is correct...watch your manifold pressure to see how quickly or slowly gas pressure rises once the valve opens and whether it holds steady at the proper pressure.
9)Also if the furnace was converted from Natural to LP make sure the correct orfices were installed.
10)Sometimes due to freezing ambient temperatures the water that is in the gas supply can freeze up the regulator and other components causing gas flow/pressure problems. This will be obvious if the problem goes away during milder temperatures.
PS...IF YOU ARE A HOMEOWNER...DO NOT FOOL WITH IT!!
Your heating system will become the most valuable asset to you on those super cold days and if you turn on your heat and nothing happens you will have what we can in the heating industry a “no heat” situation.!!
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