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Troubleshooting often requires meters and test equipment.Do you have a multi meter for testing voltage? If not you can get an inexpensive one at Walmart for about 15 bucks,
if you have or would like to obtain a meter to troubleshoot , I will be happy to guide you.Without a meter , there are a limited number of things we can check if you are able to do a visual inspection and can do some hands on troubleshooting.Please let me know and we can go from here
Excellent.Lets start with the most simple and go from there. First make sure the gas is on to furnace and the little lever on gas valve is in ON position.(I know it sounds elementary, but not unusual to be overlooked)
Excellent. Now lets turn thermostat to call for heat and check at gas valve to see if we are getting 24v ac to the valve
Absolutely.Just come back to question when ready to continue.
If we are getting 24v to valve issue is going to be with gas pressure or the valve itself.If not we will go from here by checking to see if maybe there is a limit open keeping valve from getting signal.
If gas pressure is ok to valve, then valve is bad and yes will need to be replaced.
Have you cleaned the burners ?And do you hear or smell and gas when valve gets the 24 volts?
Next step will be to verify inlet gas pressure.
You will need either a gas pressure tester or a manometer that reads in water column, There is an inlet and outlet hex screw test port.
Let me know what you find.If natural gas we should have about 5" to 7.5"WC in and 3.5"WC out of valve. If LP you should have 11.5 to 14"WC in and 10 to 11"WC out.