E97 error message

[Kelly0812]

Hi, we work up this morning to no heat and all three of our Nests reading an error E79 message.  We checked both the wires in the nests as well as our breaker box and heating system.  We still have hot water just no heat.  Has anyone else run into this or have any idea what could be wrong and how to fix it?   We’ve had the Nest thermostats for about a year and a half and haven’t had any issues until now.  

[Houptee]

You put two different error codes in the post. E97 and E79

E79 - No power to C wire detected. The Nest Thermostat is physically detecting a wire in the C connector, but isn’t
detecting any power to it.

Go to the C wire at the heating system and check it is tight and if you have a mutimeter test for 24 to 30 volts AC from R to C at the heating system. You can buy an inexpensive meter at walmart or any hardware type store for around $20 just make sure it is set to test AC volts not DC and put Red test lead on R wire and black lead on C.

Then pull nest off the base and test same wires at the Nest for power on R and C.

E97- Your Nest may have tripped an internal fuse and Nest needs to be replaced. The Nest Thermostat either has wires that aren't fully inserted into the connectors or has tripped an internal fuse. 

This sounds unlikely since all 3 are down unless the transformer is defective and putting out more than 30 V AC or there is a dead short on W heat Y ac or G fan wires which would blow the Nest fuses in all 3.

[johnCNA]

Do you have a multimeter to measure voltage?  You should be seeing approximately 24v AC between the R and C terminals.  There is usually a mini fuse on the furnace board for the 24v circuit, too.  Mine looks like a standard automotive fuse. Easy to pull, inspect and replace if necessary.

[Jeff]

Hi everyone,

In an effort to keep content fresh on the forums here and prioritize current issues, I'm going to go ahead and close up this thread. If you have any outstanding issues from this post or any other problems, please feel free to open up a new discussion.

Thanks,
Jeff