Need Help with Odd! Furnace/Thermostat issue.

Aug 20, 2003
I know we have some really smart HVAC people here. I am not HVAC knowledgeable but have a decent multimeter and tools.
So I'll try to keep this clear and not a book.

Have nest gen 2 thermostat, Furnace is Carrier 58HDX060 - - - 10112
Thermostat works fine 95% of time but it drops out occasionally going offline for a few mins at a time.
C wire not connected.
It siphons power somehow but very little. Troubleshooting says connect a C wire for more stability.

I watch some youtube videos tell the wife "Hold my Beer"
Proceed to take the furnace panel off.
Clearly marked C terminal.. has 1 wire on it from the A/C wiring (2 wires total one going to C)

I Find the Blue "C" wire from thermostat wiring bundle connect it to the C terminal on furnace.

The thermostat now reads the C wire and tells me its getting 100ma power.
Fan and AC work perfectly.(was hot yesterday)
Thermostat did not noticeably go offline(you dont know until you adjust temp from phone and it says "nest offline 12min ago" etc)

Today it says Problem with "White Wire" and heat wont kick on.

I go downstairs remove C wire from C terminal.
I did reset the furnace with breaker
Furnace works fine now.

Any idea what the issue is?

Seems bizarre.

I would really like the C wire operative for the thermostat to get all the power it needs and possibly not go sleep mode or whatever its issue is with going offline.

Edit: wiring pics adding pics



The red wire on Y and the White on C are from outside A/C

Paging @KJSmith
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Ok so I did some farther troubleshooting.
When the blue C wire to thermostat is connected I get an Error 52 overcurrent on the nest thermostat problem with white wire.
when triggering heat.
but ac and fan work.

When the blue C wire to thermostat is disconnected everything works. except it gets 40ma instead of 100ma to power the thermostat.
Shows power from C wire.

shows wiring detected.

Error 52 overcurrent (gen 2 nest error code)

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So I measured from R to C and its 28.44v no load.
Thermostat is saying 39v? is this a bad thermostat?
it says even higher voltage with No C wire ie 39.5v
sorta odd how everything works when C-blue isnt connected.
and only heat doesnt when C-blue is connected.
I need another nest to troubleshoot?

Possibly bad thermostat wiring?
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38.47v when it should not be any more the 24v?
Are you sure you hooked the blue to the c, not w?
38.47v when it should not be any more the 24v?
Are you sure you hooked the blue to the c, not w?
absolutely sure. I tried it a second time same result error 52 nest.

with meter Red to ground and red to C are both 28.44
C to ground is 0
White has never been touched.. and works fine without C-blue being hooked up to thermostat.

Something funky is going on.. and im not sure where to go from here.

I can just leave well enough alone and it works except for the one or 2 times a day it goes to sleep.. for 5-20min.. or until i walk to the thermostat and wave my hand in front of it.. so it wakes.
are you saying the thermostat is going offline from the internet?

Or is it going off, and shutting off and the unit?

If the first is true, my honeywell wifi thermostat is very picky about DNS and it wants google DNS resolvers, or it goes offline.

If you have something else configured in the home router try
Maybe the blue and white wires are touching somewhere. Thermostat cable may have been damaged during installation and caused that.

You have another unused wire in that thermostat cable, try using that one instead of the blue wire.
i would pull the white wire and reinsert it. it might be moving when you snap the nest back on the wall.
If it’s disconnecting from the app, mine was doing that and is what ultimately caused me to replace it with a Honeywell T9.
24 volt thermostat circuits including the C wire, are not supposed to be grounded. A short to ground anywhere will cause strange things.

This does sound like there is a short from the blue wire to the white wire which is likely why the common circuit was left disconnected. Try using a different wire for C if you have one. The standard color code is a black wire.

On a heat pump, the white wire circuit has a parallel connection to outside so the outdoor unit can fire up the auxiliary heat while running a defrost cycle. That can be disconnected to attempt to isolate problems with the white wire.
its a basic 1150sq ft house with basic furnace and central ac.

By offline the app and internet.. it goes to sleep for lack of a better word..
it will start working 5-30min later.. or if I walk in front of the thermostat to wake it up.

The white wire isnt moving it works fine without a blue c wire hooked up.
(also already removed and reseated all the baseplate wires.)

My 2 top choices seem to be damaged blue wire (which would also be damaged white wire.. seems unlikely but possible.

Also possible bum thermostat. It is saying the voltage is 39 and 40volts.. when its 28.4v
and the error is white wire problem e52
e52 is overcurrent

but without C wire everything functions no errors.

house came with dumb builder grade thermostat(with 2aa) and no c wire hooked up.
I dont see any more spare wires at the thermostat, but there might be a brown one somewhere in the wall. from the looks of the bundle inside the furnace.

About the only other thing I could do and it would be a major pain in the rear would be to hook up the thermostat in the basement with short wires.. and see what happens.
that would also require removing the backplate from the wall.

I will probably just buy a new thermostat for troubleshooting. if I get same issue I'll know its an issue with the thermostat wiring most likely?

Given the size of your house I'm guessing the wire run isn't long and might be pretty simple to just try fishing a new wire before new thermostat?
Given the size of your house I'm guessing the wire run isn't long and might be pretty simple to just try fishing a new wire before new thermostat?
if needed. most likely remove a few staples in basement and pull new wiring with the old wiring.
Not doing it as a troubleshooting step ;)

its 5+ years old now still works "fine" without a C wire except it randomly goes to sleep occasionally.
and says my 28.4v is 39.5v on the readout(that right there seems odd)

I also just remembered when I bought this thermostat it was not sealed. Got ninja Open-boxed by lowes..
wonder if it was returned because it didnt work on someone else's C wire.
Was mad but 40min of driving to return and it worked.... also the closer lowes was Out of stock.. and I paid with lowes GC.

I wonder if the white and blue wires are shorted somewhere(for example if someone ran a staple through the wiring)
.. when it triggers heat with the White wire... if they were shorted the blue wire should have voltage on it? hmm... IF I ground the blue wire to the C terminal with meter to check?
would that prove or disprove the damaged wire theory?

or I could DisConnect the white wire from the furnace and check continuity on the blue and white wires?

Might have to think on this some more.
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Ok Update.. blue wire is clearly bad and shorted/connected to the White wire in thermostat bundle.
disconnected the white wire at furnace.. checked ohms from blue to white.. and got... 1.2 ohms.. that is continuity.
probably the donkeys who did the remodel before I bought it missed stapling the wire bundle and put a staple through the wires would be my guess.

Found the light brown wire.. but its never that easy...
Dingdongs cut off the brown wire flush with the wiring jacket at the thermostat
no slack to pull out 3" more and restrip.
(also why I didnt see the brown wire initially.

So to fix it temporarily I cut off the blue wire and used that with a wago clip to extend the brown wire.

Had to slightly enlarge/square the hole in wall with a drillbit so the wago clip would nest inside wall far enough to get thermostat on.

Everything works now..

And I guess I'll be replacing the wiring.. its on the list of TO DO now.

/ugh :poop::poop::poop::poop::poop::poop:

At least I already have the proper wiring clip/holder and wont be stapling it to a joist like they did.
How long is the run from the Thermostat to the Boiler switching relay?
Why don’t you just re-run new wire using the old wire as a pull?

Oppps! Just re-read post #14.
It might be firmly secured in place with the staple that's shorting out the blue and white wires...
i'm hoping the last staple is the one near the ceiling in the basement and none in the wall. I should be able to confirm by wiggling upstairs while I look downstairs.

This isnt a priority right now have about 10 things to do on the burner.. but I will definitely get on it asap.. also need to measure out the run. I think 50ft would cover it but with running it around corners and up and down... not positive.

looks like 50ft of 18/7 is around 30$
Cabinet ground is the same as c. To unless you have a 5 wire two transformer system ( trans former in condenser and one in furnace ) there is no way to have 38 v. Two white wires is out of the ordinary. Trace out the white wires one is going with the thermostat bundle the other out to the condenser I think.