Furnace won't light every morning. I have the Trane mod#TUD120R9V5K0. Power cycling the furnace alone doesn't work. Tapping on the gas supply pipe and then power cycling works sometimes. Wiggling or unplugging/plugging the white 3 wire plug on the gas valve seems to work the rest of the time.

I paid $75 service call to have a licensed HVAC guy check everything out. He was here for about an hour and tested all the voltages, connections, etc and couldn't find a problem. We couldn't duplicate the problem as it was the middle of the day and the problem only happens first thing in the morning.

Any ideas?

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With a variable speed blower and 2 stage gas valve it could be any thing. When the system has faulted out it will give you a code on the control board(flashing lights). The code to the lights should be on the blower door. If we can narrow it down to the fault we can help you quicker. Flame sensing fault, open pressure switch, high limit switch and so on. All are different problems and have a different solution to the repair.

I have the same issue, but mine works fine during the day and stops making heat at night--so I get to wake to a freezing cold house. I've narrowed it down to the gas valve, the thing pictured above. I've found that when I tap on it very lightly with a screwdriver handle I can get the gas to come on. What happens is the igniter glows and after about 10 seconds there's a little click when the gas valve should open, but it doesn't. If I time my light tap on the right side of the gas valve, not the pipe, just when I hear that click, I can get the heat to come on every time. The problem is, I've got to go and do it and my house won't heat up until I've already gone to work. How do I find this part, is it expensive, and what should it take/cost to replace?

I don't recommend homeowners replace gas valves. There is too much that can go wrong with possible drastic results.