I have a 10+ year old gmnt080-4 ga furnace and have had two repairmen to the house. The issue I have is the heater will not start. The electric glow igniter works and lights the first burner but fails to light the remaining three so the flame sensor does not see a flame and the gas shuts off. If I light the remaining three burners when the first burner lights all four burners will stay lit and warm the house. The first and fourth burners appear to be burning correctly with a strong flame. The middle two flames appear to be weak as if not getting enough gas and pressure. Both service men said I need a new system and one initially said a bad gas valve but later said the value was fine. Any suggestions?
Thanks,

Yes If you have in shot burners , ( they are the short ones ) . The burners are bad, if you were to clean them that would not help. That is how they fail, one will light but not the others. This is why the manufacture installs the flame sense at the opposite end. I would replace the burners not the furnace, Make sure that your heat cell are not cracked, turn off gas to furnace , start indoor blower, use a cigarette lighter @ each cell, make sure it does not blow out. Hopefully this will help.

I beg to differ with edward3. I've cleaned hundreds of inshot burners. If these burners look like small jet planes, remove them & clean the slots in the "wings". A blade type feeler gauge works well. Chances are, they are clogged with rust or other debris. These "wings" carry the flame from one burner to the next & commonly get fouled.