E79 after disconnect and reconnect

[FrankNest]

I have had the best 3rd gen learning thermostat for years and it works fine, until I decided to change the cover plate. 

I disconnected the wires and added a back plate, then reconnected exactly as I photographed prior to disconnecting. Now I’m getting e79 errors

ive checked the connections and retried them several times and it still show the error. Flipped the breaker off and disconnected and reconnected and still th error. 
tried holding the nest button down to restart it and the error disappears, but the error shows on the app. This doesn’t make any sense since it is just out of the blue.

[sterlinghawk]

• Is there a reset button?

[FrankNest]

Reset button on what?

[sterlinghawk]

If I understand correctly, the Nest thermostat is working but showing error in the app

[FrankNest]

No, the heating will turn on, but not the AC. And the thermostat shows an error as well as the best app, both a wiring report e9 no power to common (C) wire detected

[DaveLister]

Check inside your system to for a blown fuse. If you didn't turn off the power to it when you added the back plate, it's possible the C wire touched another one and caused the fuse to go.

[FrankNest]

Where would the fuse be? In the ac unit or the air handler?

[DaveLister]

Wherever the wire from your thermostat goes to, which would likely be the air handler. I had the exact same problem a couple of years ago and it turns out there was a fuse on the C wire that had blown because I didn't think to shut the whole system down while I was trying to install the thermostat.

[FrankNest]

Ugh, that’s in the hard to access attic with blown in insulation. I have a fun day ahead of me! 

is this a special type of fuse? Hoping to have the replacement before I venture up there.

Just had the system installed 2 years ago.

[DaveLister]

The type of fuse will depend on your system. If you don't have easy access to a manual or something with the model number to look it up, perhaps your installer would have that info on record and could tell you exactly what fuse and where to find it.

[FrankNest]

Super helpful, thanks DaveLister!

was changing the fuse relatively simple? On a scale 1-10, 1 being changing a light bulb and 10 being changing an outlet receptacle, what would you say the difficulty is?

[DaveLister]

Changing it was easy. In my case, it was an automotive-style blade fuse in a holder like this:

But not having any information about your system, I can't say for sure that it'd have the same kind of setup.

[FrankNest]

Looks like a 5amp automotive fuse will be what I need. Found the model: Carrier FV4C. 
going to give it a go this weekend

[sterlinghawk]

I just did it, be careful, I tripped and broke a rib

[FrankNest]

Oof, sorry to hear that!

[Markjosephp]

Hello folks,

I wanted to stop by and see how I could help.

Thanks for the help here, @DaveLister and @sterlinghawk.

@FrankNest, in addition to what was suggested above, could you please gather your thermostat's power reading in the Technical Info option? Does it have a C-wire or a common wire? If there is, try removing the common wire and leaving it disconnected, then restarting your thermostat to refresh.

Note: Turn off the power from your HVAC system at the breaker box before touching the wires.

Let me know how it goes.

Regards,
Mark

[Dan_A]

Hey there,

I wanted to follow up and see if you still need help. Please let me know if you are still having any concerns or questions from here, as I would be happy to take a closer look and assist you further.

Regards,

Dan

[FrankNest]

@DaveLister  was right on point. The 5 Amp fuse in the air handler was blown. Replaced it and we are good to go!

thanks for the follow up

[DaveLister]

I'm glad I was able to steer you in the right direction! We won't talk about how I knew that was a possibility... 🤣

[Markjosephp]

Hello there,

I appreciate you posting and updating us on your efforts to address your concern and find a fix. Hats off to both of you!

It looks like we have come to a solution, so I'm going to lock this thread in 24 hours. If you have any other concerns or questions, please feel free to create a new thread.

Best,
Mark