This afternoon, our Ruud furnace suddenly stopped working. After poking around a bit, I realized that the 3A fuse on the control board had blown. I ran to the hardware store and replaced it, but no luck... still not working.

I looked around a bit online and found a lot of references to the thermostat wiring as a possible culprit, and a few minutes with the multimeter later thought I'd found a couple wires that might've been shorting together. Luckily enough, the installer has used a cable with a few extra, unused wires, so I tried connecting the thermostat with those instead. NOPE.

At this point, I've followed some other advice I found and disconnected ALL the wires from the thermostat and condenser from the furnace control board, but the 3A fuse STILL blows the moment I press the blower door switch. I'm pretty much at a loss now... Any ideas?

I'm not certain of the specific model, but it's a Ruud furnace/AC... this is the manual I have for it (ours is an upflow model).


  • Did anything change, or did this happen randomly out of the blue?
    – Tester101
    Nov 8, 2015 at 3:10

1 Answer 1


If you have disconnected all external wires to the control board with only internal wires to the unit connected and the fuse still burns, that very likely indicates a failure of the board. With the fuse burning quickly, that often means a component in the power supply side of the board has catastrophically failed. To verify this, you could disconnect all wires to the board except for the power connection and the door switch and test to see if the fuse still burns in that condition (make certain you are using a correctly sized replacement fuse, obviously).

Replacing a board is typically not difficult if you can find a compatible replacement. Unfortunately, the wholesale price for these is often $200+ or higher.

  • 100% correct. Thankfully, we'd paid for an extended warranty that covered the cost of the board. Annoyingly, they wouldn't cover the diagnostic fee charged by the HVAC tech to come out and do exactly what I'd already done, but still... $95 was better than a few hundred.
    – Justin S
    Oct 9, 2016 at 23:28

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