So I have been having a bit of an issue with my furnace since installing the nest thermostat. The first day after I installed it was perfect. Everything was working. The second day I kept getting the error that C wire had no power. I redid all the wires on the furnace control board and the thermostat (cut and restripped them). That got it working for the next couple of weeks. However, the last 3 days I have been having other issues with it. About 3 days ago I had an issue with no power to the RH wire. I checked the wires, circuit breakers, and the fuses and all was good. I went to check the voltages R to W good, R to Y good, R to C spark and burnt fuse (was following a nest troubleshooting guide). I replaced the fuse and miraculously the nest can detect voltage on the RH wire. Today, I come home from work and nest is displaying the no power to RH wire error again. I check the circuits good, wires good, voltages showed 0 (I did R to W and R to Y). I even jumped the R to W and R to Y and nothing. I checked the fuse and it was good good (removed it to get a better look at it this time). After I put the fuse back in I jump the wires again and another miracle the furnace starts. I have an older york furnace (1990s) and it has been pretty good until now. Any help to figure out a fix for this issue would be greatly appreciated. The wires I have for the thermostat are RH, W1, Y1, G, and C. On the furnace control board they are in this configuration.

    Y       W   R   G    C          (Board labels)
Y(t),Y(ac)  W   R   G  C,R(ac)   (wires in each slot. t=thermostat, ac=ac unit)
  • You said you blew a fuse when checking voltages. Volt meters usually have at least 1m ohm / volt, the impedance is so high this can not happen. If you bridged some terminals or had the meter in the wrong mode it could happen. You said you checked wires and they are good what is good? A voltage level would be appropriate to state here as 12 v may look good but 17v may be needed. I would be looking at transformer output voltage. – Ed Beal Feb 20 at 1:43
  • When I say I checked wires I mean I made sure they were seated properly. When I checked the voltage, I checked it with a multimeter in 200V ac mode. I put the red probe on the R and the the black probe on the W, then the Y, then the C. On the C the 3A fuse on the control board fried and a small spark happened when I touched the terminal with the probe. It never read any voltage on the R to W or the R to Y when I checked it those 2 times. However, Right now it is reading 24V. – Fireurza Feb 20 at 2:36
  • What make and model is your furnace? – ThreePhaseEel Feb 20 at 3:52
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    The 24v is what I would expect, I still don’t know how you would pop a fuse taking voltage measurements unless you shorted things with the probe. With an intermittent voltage I would be looking to find a relay with bad contacts. Measurement voltages should be made with reference to the common c wire. – Ed Beal Feb 20 at 11:25
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    @ThreePhaseEel it is a York Model G8C07516MUC11A furnace. – Fireurza Feb 21 at 2:22

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