What I know- it's not the thermostat or the blower. AC and fan work. Furnace will not fire.

I have a multimeter and I know how to use it, but zero experience troubleshooting HVAC equipment. Any tips where to start, or should I just save myself the trouble and call a pro?

Unit is a "SX Ultra 80". That sounds like a model rather than a manufacturer to me. I dunno.

Few pics-








I have a feeling this might be the problem right here, but am at a loss as how to check.

Tim, most likely that IS going to be the problem and there is no way for you to check it out. Basically when components like that go south, it's a matter of replacing the whole printed circuit board. There's no enough info to say for sure what that board does, but more than likely it's involvement is going to be safeties that keep the furnace from firing it they are not met. Does the inside cover of the furnace have a schematic drawing glued to it? If so, at least you might be able to easily get some info about model and brand and so on. Good luck.

I had the same problem last year on a gas furnace... I will look at the part I replaced in the morning... too dark now..

Does the exhaust motor come on?

Sequence of operation is as follows:

Vent motor

Pilot ignition

Main burner ignition

Blower comes on.


The circuit board (which even if it isn't the problem I would replace and repair the water leak) sends signal to the gas valve. The gas valve tells the exhaust motor to start, then light pilot etc...

If the exhaust motor does not start it's most likely the circuit board. Disconnect the plug going to the motor and see if there's 120v going to it. If there is your motor is shot.


If it starts check the presure switch ( the round metal disc behind the gas line in the left corner of the top pic) It's contacts should close when the motor runs. It that is good then most likely your gas valve is bad.


It could also be the heat contacts in your Thermostat are bad. The A/C and blower will come on but no heat. With the stat calling for heat you should get a 0 volt reading between the R and W contacts on the board.

Make sure the door switch for the blower compartment is pressed in when doing this test. Stick a piece of duct tape over it to hold it in.


I'm putting my money on that crusty ass circuit board.




Oh yeah IS THE GAS ON:icon_lol:

Thanks for the input, guys


I'll go down the list as suggested and report back, Brett. The gas is on (stove works fine and I certainly didn't turn off the valve at the furnace) and the thermostat has been replaced. The old one was a little arc scarred and dirty looking at the heat contact points- I had hoped it would be that easy, but no such luck.

No power to the exhaust motor. Looks like I'll be pulling that circuit board and looking into replacing it. Nothing going to happen until tomorrow beyond removing the part. I'm not paying emergency rates just to be told the part will be ordered tomorrow.

Circuit board removed and looking more and more like the culprit.

Tim, it is possible that the board is still good. Most times the corrrosion crap is very conductive. Take an old toothbrush, and use some alchohol to scrub off the corrosion. You might be back in business, it is worth a try.

Oh, and it is in the 80's today out here

That occurred to me (you can see in the pic that I had already started cleaning up around the resistors in the corner), but I discarded the idea thinking maybe after being shorted whatever is in that chip might have fried. I also wasn't looking forward to hooking the thing back up in that rather cramped space for possibly no gain, then removing again- feeling lazy. Now that you've suggested it I guess I'll give it a shot. I'm not getting a new part today either way, so it's my only chance for heat tonight.



Can I send my family to your house until I get this sorted out?

No joy, but I'm now convinced it is the board. Having dismantled it and put it back together, I was in a better position to do some testing with it installed and powered up. I've got power to the board, but nothing going to the exhaust fan with the thermostat telling the unit to heat.


Just a matter of finding the part now. Aint' gonna happen today, so I guess I'm done kneeling by that damn thing for now.

If they don't mind mowing grass between beach trips

New control board, $73 and change. Installed, up and running, Done.


Thanks for your help, Brett.

No problem!