I'm trying to find out why my furnace won't ignite. The pilot light is on, I can hear something turn off when I checked the breaker and it came back on when I turned the breaker back on. If I turn the fan to on it works. I'm thinking it's the thermo coupler but I don't know how to test that. Can anybody help me please?

Switching the fan to on at the thermostat and the fan coming on tells us the control circuit/transformer is intact. The pilot light is lit (per your post) so we can rule out the thermocouple for the most part (it can be tested, but costing less than 10 bucks for the part, I would replace it if questionable, and total failure would result if it were blown). The test would be to see if you're getting 24-26 volts AC at the gas valve, if so-the gas valve is bad. If not, check and see if you have 24-26 VAC between W and C terminals on the terminal board in the furnace (may have to tape down the door switch to get unit to run with door off). 24VAC between W and C terminals means the thermostat is doing it's job, and we can rule that out. If you have power (24VAC) between W and C, and no power to the gas valve, you probably have a safety open, and I strongly recommend you get a professional to come in and resolve the problem, as something has caused the safety to trip open. Open safeties can be anything from a clogged filter to a serious problem, and requires PROPER resolution-not just making the unit come on again.

In all honesty, I think this should be personally, physically investigated somebody that knows HVAC/R, especially if you're not comfortable using a meter.

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Forget the thermocouple, the pilot is on. Do not try to operate the furnace by-passing the door switch!! You can be injured. Look for 24 volts, your transfo can be out. Because if you have a thermocouple(first generation furnace, you don'n even have a board. I imagine that you turn the fan on with a switch or in the high-lim device(whitwe knob)

The pilot lights, but when release to turn knob to light the flame goes out. I changed the thermo coupling

Mistake I meant boiler type hydrotherm not furnace