I have a Concord 90 Plus furnace. 90UA075D12B. About 7 years old. It has lately not been firing up. The fan will run. If I turn off the power and back on again it will run for a cycle or two.

Could it be the thermostat? It is older than the furnace. The thermostat light will come on indicating the furnace should be heating. But the furnace will not fire up. If I switch the thermostat from "heat" to "off" to "heat" again it will fire up and work for awhile.

Any suggestions would be greatly appreciated. Thank you in advance.

Possible a weak transformer,, do you have a meter to check how much voltage the transformer is putting out,, this is the first of several things it might be

Have you actually watched the furnace to see what is happening? Do the burners lite at all, even for a few seconds? May be as simple as a dirty flame senser. Let us know more and we will do our best to help.

Hello and thanks for the quick responses. I don't have a voltage meter, but I did look up a bit more on the flame senser. I found that easy enough and it did have a bit of white "corrosion" on it. I cleaned it with some steel wool and will watch to see if it fires up on the next cycle. I have been watching and it would work fine and get up to temperature when I reset it. So I will have to see whether it will come on the next time it is needed without resetting.

I will keep you posted. Thanks again to both.

Thanks to to you both again.
It's come on now three times without being turned off first. It has got up to temperature each time. It must have been the sensor. Thanks for taking the time to post. My wife and two young girls are thankful as well. We have a frigid -12 C or 10 F night here in our little Ontario town. I don't think the electric portables would have kept up throughout the night. I would have been staying up to keep resetting the thing.
Thank you!

Glad its working so far. Take care.