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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?

  • 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 5 '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 5 '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 5 '15 at 11:44
  • Thats what happened. I dont understand. – Billy Korsen Aug 5 '15 at 11:54
  • You mentioned a 5 amp fuse, but I'm seeing it as a 3 amp fuse in the installation manual 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 5 '15 at 12:02

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