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Hello,

Im looking for some suggestions please. I have an older furnace and the burners wont ignite. Initially, the pilot wouldnt stay on. Changed the thermocouple and that didnt work. Then we replaced the gas valve. Now the pilot will stay on, but the burners wont ignite. We have replaced the thermostat and changed the filter (which had obviously not been changed in years, and hopefully didnt crack the heat exchanger). (Also, the limit had been tripped). We also replaced the transformer. My stepdad jumped the gas valve and the burners came on, but he immediately removed the jump (just trying to see if it would work). The gas valve currently on there is a little bigger than the old one, but I was told that they were all regulated and cant be too powerful. Any suggestions?
 

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I'm Your Huckleberry
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you need a manometer to test gas PRESSURE across the gas valve. is the valve being sent 24 volts to open? DO you hear a distinctive click at the gas valve indicative of 24 volts being sent to the valve? that would happen upon a call for heat.

post the model and serial number of the unit. is there a control board which sends 24 volts to the gas valve? if so test on the board at gas valve output to see if 24 volts is even being sent to the valve. if not, new control board.
 

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A few thoughts..
Gas pressure, whether too high or low, is unlikely to be the fault if a jumper enables the gas valve to fire.

If you don't have an electrical meter, check for secondary limits or roll out switches that may have reset buttons. Follow the two wires connected to the gas valve, that your jumper bypassed and got the gas valve to fire. The problem will be somewhere along this route.

A meter will enable you to follow this route to see where the 24Volts is not completing the circuit to get the gas valve to fire.

Posting photos of your furnace with the doors off will help us figure out what your dealing with.
 
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