Furnace issues, suspecting control board

[96Firebird]

Tried to turn on my furnace this morning to make sure it is all working well before winter starts. No heat. No fans or nothing. I suspected it could have been my Nest thermostat, but it worked fine last winter. I hooked up my old thermostat and same issue, nothing happens.

I hooked the Nest back up, tried fan only and it the fan fires up. Works fine with AC too... Check furnace error code, 3 blinks meaning pressure switch error. Well the pressure switch isn't moving because the inducer motor isn't spinning. Checked voltage at the inducer motor, nothing. Checked voltage at the circuit board for the inducer motor, nothing. So I think my control board is shot, but wanted to see if there is anything else I should check before replacing that. I figure if there isn't 120V coming out for the inducer motor, there really isn't much else it could be...

 

[mindless1]

How much time elapsed between the initial problem and your testing? Some furnaces will go into lockout mode for a fixed period of time, and some might stay in that mode unless you do a power cycling. If you have not power cycled it then I would try that next.

After a power cycling, have someone watch the intake air blower to see if it spins while another turns the furnace on. It could be it is working but is the pressure switch itself, or it might have even went further in the cycle and failed to ignite then went into lockout mode.

If you have basic electrical skills and don't feel uncomfortable working with live AC circuits (though some tests wouldn't require live power) there may be some things you could test on the control board, like whether a relay is getting coil power but not activating but I'm getting ahead of myself if you do a power cycle and find that it locks out at some point. Many faults will cause a lockout before the main blower comes on.

It might not hurt to mention the brand, control board model #, maybe even a good, same orientation, high-res, top-down and bottom-up at 90' angles (lol, so components on front can be matched to traces on back more easily) pics of the control board.

 

[bbhaag]

The pressure switch operating properly depends on the exhaust motor spinning correct? The exhaust motor creates suction within the switch completing the circuit. So I agree with mindless1 about having someone else turn on the furnace while you watch the motor and see if it powers on and continues to stay on while the rest of the system cycles.
If the motor powers on and continues to spin at the factory rpm speed then that is good. Make sure the motor is spinning at the recommended speed. If it is burning out or seizing up it won't create enough pressure to compete the circuit inside the switch. If the motor checks out then it would be time to check out the pressure switch. It has been my experience that these rarely fail though.
If it is the switch a quick and dirty method I've used for testing is to pull both wires off the contacts and then use a paper clip to complete the circuit.

 

[mindless1]

I would have to see the system to do more than generalize, but in general the pressure switch depends on the exhaust motor spinning, and having a reasonably unobstructed pair of pipes. Certain things like rodent or insect nests, or exhaust condensation (usually during winter) in a sagging pipe could lead to a pressure issue, as could the diaphragm for the pressure switch developing a leak, the switch itself failing, or possibly some interlink between the two (probably all come together as a set, part #).

If the exhaust motor does not spin up then I would listen for humming, measure for power to it, and possibly remove it and see if the shaft spins freely. I recall that on mine, about 8 years ago it started to develop a squeaking sound and had I not pulled it apart and lubed the bearings, it might have seized up by now. Pulling (a remotely modern) one apart is not normally considered a user servicable procedure nor one a typical HVAC tech would do, but if it is bolted or tabs are bent to hold it together then disassembly is not too difficult. Now I have gone off on a tangent.

If no power is getting to the motor initially, that usually indicates a circuit board failure, but I do not dismiss the possibility that a new(er) furnace could have some kind of initial sensor which keeps anything from starting. Often a system has a diagram sticker inside the furnace cover panel. You could post that, or try to find one online or in the owner's manual.

Some types of circuit board failure could be very cost effective to DIY repair such as a failed relay, burst capacitor(s), damaged bridge rectifier, logic power transformer with its internal thermal fuse blown (they should reset but after some age may fail instead). The list goes on but some detail to point in any direction would help.

Then again, the parts supplies have changed in recent years. Now some control boards can be found a lot cheaper on eBay or Amazon than if you had to pay > $100 more for it through an HVAC tech doing a repair.

 

[96Firebird]

Model is Goodman GMT070-3A. Original furnace from when the house was built in 2003. Here is an image:

I've circled the connectors for the inducer motor. That isn't getting 120V. I measured the input on the control board and that IS getting 120V. The output on the control board for the inducer motor is not putting out any voltage, and I checked the impedance of the wires for the inducer motor and they read close to 0 ohms.

The inducer motor spins freely, no issues with it being stuck or the bearings going bad. I was thinking about hooking at extension cord up to it to make sure it fires up, but I don't have any spare extension cords I can cut up.

I don't think the issue lies with the pressure switch, since the inducer motor isn't even turning on to build up pressure.

And yes, I am pushing in the door switch when testing everything. I usually have to wait a minute for my thermostat to turn the furnace on, but it's easy to watch it happen on my phone. Also, the control board will go from solid red light to the 3 blinks and you can hear a distinct buzzing sound when this happens.

After doing some looking around online, I can get a new Goodman control board for around $100, or get an aftermarket board for half the price. There are also used options on eBay for around $30, not sure if that is a wise idea though.

 

[mindless1]

I would try to locate the buzzing sound, take a non-conductive object like a stick and tap on the relays to see if it gets one to toggle when the furnace is trying to start. However I would expect that once the light starts blinking that it is already in lockout mode, unless it attempts to keep trying for a little longer, so I might try tapping the relays even before the furnace tries to start.

Next if desoldering and soldering in new relays is within your comfort zone then I'd get the datasheets for the relays and apply the appropriate coil voltage across each relay in turn on the circuit board to see if it closes the contacts it is supposed to. You can trace back from the output to the inducer motor to see which relay(s) are involved. If testing individual components on the circuit board is more than you're willing to do then I would try to assess whether the half price aftermarket board is decent quality and buy it if so. Obviously a used board is a gamble, no way I can assess that especially if it is a failed relay and the used one has nearly as many years wear on it already. For what seems only a $20 price difference (between half of $100 and $30), I wouldn't get a used one.

One other thing I would do is check the fuse on the board, at least in a better pic I saw it appears to have one, purple plastic in the corner, and on the transformer above it I would confirm that 120VAC is getting to it and some lower voltage AC is leaving it, though if it is the only transformer than we can just about assume it's working as that's power going to the thermostat too, and probably the board wouldn't work enough to blink the LED.

While I have it open anyway, here's a pic for others to look at a new OEM control board. Whether it is exactly the same as the old or not I cannot say, only that it is listed as a replacement for that furnace model and several others:

 

[96Firebird]

Didn't want to bother re-soldering new relays onto the board, so I spent the $70 for a new board. Put the new board in last night and everything is back to working fine... Just in time too, since it got to the mid 30's last night.

Good to know my troubleshooting didn't sway me in the wrong direction, and I didn't have to spend money getting someone out to replace the board.

And I think the buzzing sound is the transformer just above the control board.

 

[mindless1]

Yeah getting someone to come out and diagnose, by itself would usually cost more than the board if buying it yourself. If buying it from the HVAC guy, your total bill incl. labor could be $300 or more.

 

[stormkroe]

FB, you may already know this, but just incase:
short R and G to run fan
R and W for heat
R and Y for ac
You can test functions at the unit without having to run back and forth from the T-stat.
Also, be sure to check for a board level fuse before you throw the old board out, it would be a simple fix and then you would have a spare board.

 

[96Firebird]

Yeah I knew about shorting to run stuff, saw that on a website somewhere. Didn't need to do that though, since my Nest can be run by my phone I would just sit next to the furnace and change the settings from there.

I also checked the 3A fuse, it was still good on the old board.

I'm actually still not sure what is wrong with the old board. Turned out my original multimeter was failing, so I bought a new one and it was reading 30V when the inducer motor was trying to kick on. It would slowly decrease (~5V/min) the longer I waited. But that is also measuring hot to neutral, so maybe it was something else?

 

[mindless1]

30V on the inducer motor power leads? 30VAC or DC? That would be odd, since it is presumably a 120AC motor (and no way to rectify and regulate DC is shown if it weren't), and nothing should be tapping into its power feed, just a straight shot from 120VAC input to output through the relay.

Best guess is the control board relay to it had fouled contacts and the extra resistance was pulling the voltage down. If that's the case, the time-out circuit probably saved the relay from heat buildup, looking charred if not melting, but that also hid a visual hint of the fault.

 

[stormkroe]

30V on the inducer motor power leads? 30VAC or DC? That would be odd, since it is presumably a 120AC motor (and no way to rectify and regulate DC is shown if it weren't), and nothing should be tapping into its power feed, just a straight shot from 120VAC input to output through the relay.

Best guess is the control board relay to it had fouled contacts and the extra resistance was pulling the voltage down. If that's the case, the time-out circuit probably saved the relay from heat buildup, looking charred if not melting, but that also hid a visual hint of the fault.

Agreed. Sounds like a classic switching issue, most likely in one of the relays. More times than not, you can read 30-50v on the 'OFF' screw of a live 3 way switch, but there's not enough current there to do anything other than register on a meter.

 

[bbhaag]

Didn't want to bother re-soldering new relays onto the board, so I spent the $70 for a new board. Put the new board in last night and everything is back to working fine... Just in time too, since it got to the mid 30's last night.

Good to know my troubleshooting didn't sway me in the wrong direction, and I didn't have to spend money getting someone out to replace the board.

And I think the buzzing sound is the transformer just above the control board.

Good to hear man. Glad you listened to your gut and figured it out. That's a good feeling. I'm in the middle of trouble shooting a commercial Modine unit right now and it's giving me fits. It's an older 360k btu unit that just isn't cooperating....sigh.
Since you know your way around furnaces now you feel like heading my way and giving me a hand? I got beer and pizza.haha

 

[Red Squirrel]

Turn heat on, disconnect pressure switch, and see if it starts. If it does, it means the pressure switch is stuck open, basically before the inducer blower even starts it makes sure the pressure switch is closed, once the blower starts it then opens the pressure switch, and it will do a second check but to make sure it's open before it fires the burner. So if you disconnect it while the tstat is calling for heat and the inducer motor starts to spin, plug it back in, and the the furnace should start.

This may "reset" the switch and you might be good to go for a bit, but it will probably need to be replaced. Not too hard of a job.

Edit: On NM just saw you already fixed it. But figure this info might be useful for someone else in future as I had the same issue happen myself.