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Have you tried cleaning the flame sensor?
What did you clean it with, can I ask?
OK, that sounds right. Can I get full model number, please? What LED flash code are you getting when it shuts down?
Are the burners lighting at all?
When they light, watch them. Do they light smoothly and all the way across quickly?
OK, the 1 flash code is ignition lockout. There are many things that can cause this. You may have already seen this, but here are some of the possible causes of an ignition lockout.
One thing to check is that front cover pressure switch. Is the front cover on securely when you try to run it?
If it was a thermostat issue, it would NOT lockout with a flash code. It just wouldn't be on at all.
You may have a bad gas valve. Get a voltmeter and set to "volts AC" and put one probe on each of the two terminals of the valve. Watch it through a few cycles. If the valve gets voltage, but no gas, and you have good gas pressure, valve may be bad.
The valve could be getting voltage from the board, but if it's working intermittently, could cut out, hence that's why we need to confirm voltage.
Please start by checking voltage to the valve through several cycles. I want to confirm that we don't have a bad valve. What we are looking for is voltage to the valve (24 volts), but no gas coming out, and yet the gas supply to the unit is still there.
As long as there is a call for heat, you should have 24 volts to the valve. If it drops off, you may have a bad board.
You can try jumping R to W at the board after first shutting power off. This will simulate a call for heat when you power back up. The inducer should start, pressure switch will close and then ignitor will light and you will get 24 volts to the valve. This voltage should remain as long as you have R to W jumped.
It s also possible that the furnace could shut down if flame isn't sensed. Even though you cleaned flame sensor, it may need replacement. Make sure you have a good ground from the board to the burner area.
Yes, it will try 3 times, then go into a lockout for a period of time. Again this could be a weak ignitor or even the flame not being sensed (poor ground from board to burner area, bad flame sensor.). The problem with this 1 flash code is there are SO MANY THINGS it could be. You saw the list. I'm just trying to eliminate some of them.
Did you have additional questions on this?