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I came home to my furnace having no power even to show trouble codes. I turned off my breaker opened the blower door and it had a blown 5 amp fuse on the curcuit board. I replaced the fuse turned the breaker back to on put the door back on and the furnace and ac kicked back on. After a couple minutes I noticed it was back off. I went back out checked the fuse again. The fuse was fine. I went outside to try to find a short in my wires did not find anything. The furnace again has no power. The breaker is on, I have no external switch, the fuse is ok, and the blower door is back on. Also i used a multimeter to make sure I was getting the 120v to the furnace and it did. Does anyone know what else could have gone wrong?

Billy Korsen
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  • Is this when the system is calling for heat, cool, or both? If the problem is during a cooling call, is the outdoor unit running? What is the make and model of the furnace? Have you tested all the limit switches? Is the filter clean? – Tester101 Aug 05 '15 at 10:47
  • Its during both. The outdoor unit ran after the fuse was replaced till it stoped again. Its a lennox merit series g40uh I do not know what or were a limit switch is. Filter is new. – Billy Korsen Aug 05 '15 at 10:57
  • When it was working, both the indoor blower and outdoor unit were on. Then a fuse blew, and everything shut down. When you replaced the fuse, *everything* came back on again? And now *everything* is off, but the fuse and breaker are both still set? – Tester101 Aug 05 '15 at 11:44
  • Thats what happened. I dont understand. – Billy Korsen Aug 05 '15 at 11:54
  • You mentioned a 5 amp fuse, but I'm seeing it as a 3 amp fuse in the [installation manual](http://www.lennox.com/pdfs/installation_maintenance/Lennox_G40UH_IOM.pdf) for your unit. Set your multimeter to measure volts AC, and measure between the `R` and `C` terminals on the control board in the furnace. You should read somewhere around 24 volts. My concern is that the transformer was fried. NOTE: You'll have to hold the cover switch in, so the unit powers up. ***WARNING:** You'll be working in live equipment, take care not to touch anything.* – Tester101 Aug 05 '15 at 12:02
  • I got 0 volts there – Billy Korsen Aug 05 '15 at 20:37
  • You made sure the door switch was closed (pushed in), right? If so, the only reasons you wouldn't get 24 volts is the transformer is not getting power, the transformer is dead, the fuse is blown, or one of the rollout switches are open. Using your meter set for voltage, measure from one of the neutrals terminals to the XFMR terminal. You should get about 120 volts. – Tester101 Aug 05 '15 at 21:37

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