E72 error message on Nest learning thermostat

[RoyTom]

Have been using thermostat without incident for over 1 year and suddenly received error E72. All wires are connected as they have been from the beginning. I attached a photo of the wiring.

[CoolingWizard]

An E72 error means no power is coming from your HVAC equipment.  Check for a tripped circuit breaker or a blown fuse in the furnace control board.  Don’t just replace a blown fuse until you find out what caused electrical short otherwise you’ll just be wasting a fuse again.

The AC Cooling Wizard

The AC Cooling Wizard

[Jenelyn_O]

Hi there,

Thanks for reaching out. I just want to confirm if you were able to check the fuse in your control board. If the fuse seems fine, power cycling your power breaker might help. You can check out this support article for more information. 

I appreciate your help, @CoolingWizard.

Best,
Jenelyn

[RoyTom]

I disconnected the Nest and re-installed the old thermostat and it is working fine. Leads me to believe that the Nest is faulty on some level.

[Markjosephp]

Hello folks,

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

Thanks for assisting, Jenelyn.

@RoyTom, the common wire may not be properly grounded, so the system uses power sharing but doesn’t get enough energy and shows the no power wire at all. If possible, you could give it another shot and ensure that the power from your HVAC system at the breaker box is turned on after the Nest Thermostat installation, and that all wires are properly connected and the tabs are pushed down. Also, try disconnecting the C-wire and leaving it disconnected.

If the same report appears, gather the power readings from the Technical Info option.

Let me know how it goes.

Regards,

Mark

[Jake]

Hey RoyTom,

I wanted to check in and see if you managed to see Mark's post. Please let me know if you have any questions from here. I would be happy to assist, and make sure you are good to go.

Best regards,
Jake

[CoolingWizard]

I noticed that your wires are not solid cable wires are stranded wires. I’d be very curious who installed that system using stranded wire instead of using a standard industry, common thermostat cable which uses solid conductor wires.

The AC Cooling Wizard 

[CoolingWizard]

RoyTom,

I am not sure who the company was that installed your system, but there is not a proper thermostat cable being used. A standard industry thermostat cable uses solid copper wire conductors and not stranded wire conductors like yours is using. Consider this that I noticed:

In your photo if you look closely at the Rh connector of the nest back plane, the red wire is not properly connected because you can see that the plunger that holds the wire is not down. This would be reported as a E72 error.  because the Rh wire is not all the way in there properly.  When a wire is properly attached the plunger you push on the backside will stay partially down if it’s up and lined up with the others that are not using wires you have not inserted that Rh wire in properly. It also appears the stranded wire you are using in larger gauged wire than the standard thermostat cable which are 18 gauge, solid copper conductor. 

The AC Cooling Wizard

[RoyTom]

This is the existing wiring in our 1973 built condo. The Nest worked fine for a year and a half so I'm not clear as to why it would suddenly stop working and give me an error message. I didn't change anything. I disconnected the Nest and reinstalled the old thermostat (a conventional one) and everything works fine.

[CoolingWizard]

The picture that you showed make shows Lee wire is not properly inserted so maybe it’s barely touching the connector inside, but it’s not in the pitching terminal that holds it in tight. So it may have worked slightly for a while, but during time of heating and cooling the wires, expand and contract and that wire pulled away. That’s why you got the error. You need to remove the wires cut the crumbled up wires off restricted properly twist wires to hold them together and then ve push all the way down on the retainer button and gently slide the wire in to at least a 3/8 of an inch. 

The AC Cooling Wizard 

[Markjosephp]

Hello everyone,

I appreciate your continued support here, @CoolingWizard.

@RoyTom, I'm checking back in — how's it going? Still need our help?

Regards,

Mark

[RoyTom]

Thanks for all who tried to help. I removed my Nest and reconnected my old conventional thermostat and have not had a problem since. I returned the Nest to Costco and got a refund. I’m disappointed that I it failed to function in my condo in FL. I still have one in my home in NY state though, and will continue to use it. Someone suggested that it was not connected properly but that was not the case. I checked all of the connections when I was troubleshooting. It worked without any issues for over a year so at the end of the day I’m still unclear as to why it suddenly was failing to get the power it needed to function. Thank you to everyone who tried to help!

[CoolingWizard]

As I stated before, your red wire was not properly inserted in that back plate so the problem was really the types of wire they’re being used, and the fact that the wires were all scrunched up like a bird nest, instead of being kept straight and fully inserted for the instructions of the back plates. I wish you the best. 

The AC Cooling Wizard

[Jenelyn_O]

Hi there,
 

You can refer to the responses of CoolingWizard above. Please let us know if that will work. You can also visit this support article for additional information about E72 code.
 

I appreciate your help, Jake, Mark, and @CoolingWizard.
 

Best,

Jenelyn

[Jenelyn_O]

Hi RoyTom,

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

Thanks,

Jenelyn

[Jenelyn_O]

Hello there,

We haven't heard from you in a while so we'll be locking this thread if there is no update within 24 hours. If you need assistance or want to discuss topics, feel free to start a new thread.

Regards,

Jenelyn