Nest 4th gen AC shutting off after a few seconds

[RicardoF]

I just installed a 4th gen Nest and now my AC unit turn on for only a few seconds (30 to 40 sec) and then shuts off.  My wiring is Rc, Y1, W1, G and C. The power readings on the thermostat are Vin 31 to 33, lin 300mA, PS C-300.

My previous 2nd gen Nest never had this problem and was working fine, but unfortunately will no longer be supported. Any ideas what could be causing this and how to fix?

[JuanCH]

Hi RicardoF,

Thanks for reaching out to the community! I understand you're having trouble with your new Nest Learning Thermostat 4th gen, where your AC is turning on for a very short duration, only about 30 to 40 seconds, and then immediately shutting off. I appreciate you sharing some wiring details and power readings. This is definitely not the experience we want you to have, especially since your Nest Learning Thermostat 2nd gen never had this issue. I'm here to help!

To isolate the root cause of this issue, I have some questions for you: 

• Have you experienced any power outages within the last few days or weeks?
• When was the last maintenance check for your HVAC system?

Please keep me updated.

Sincerely,
Juan

[RicardoF]

Hi Juan,

Thanks for your reply. No power outage issues. I just had an HVAC technician do an inspection and the AC unit is working fine. He also couldn’t figure out why the thermostat is acting this way. 

[JuanCH]

Hi RicardoF,

Thanks for keeping me updated! To continue addressing this issue, could you please provide a screenshot of your Nest Thermostat's energy history from the Google Home app?

You can find this by following these steps:

1. Open the Google Home app.
2. Touch and hold your device's tile.
3. Tap on "Energy dashboard."
4. Select "Week."

I look forward to your response.

Regards,
Juan

[Karlef0714]

Have same issue, moved from Nest 1 st Gen to Nest 4th everything works until it cool turn on works 30 sec then turn off/on about 1-2 seconds and then run again and then it does it again, I am afraid of damaging my AC, just installed 24 hr ago wire is the same Y1,G,RH,W1

[Cheezer12]

I also have the exact same issue.  "Upgraded". from Nest 2nd gen to 4th gen yesterday and when thermostat calls for cooling the compressor only comes on for about 2-3 seconds.  The 2nd gen and AC system worked fine yesterday prior to upgrade.

[Karlef0714]

Called Nest support no help,they blame my AC unit you didnt do the annual service thats why is happening he said LOL. I told them what about AC for 10 years was fine and just yesterday that I installed the nest is happening?? I found the issue myself, I am no technician at all. The problem is that new Nest need more power and need C cable connected as well with other cables. Mine had one connected with AC unit outside, i shared the cable to fees Nest thermostat and it works perfect like old one. Find C cable, if you dont have one need to add it otherwise the problem will stay and you will damage AC unit. Thats the issue that no one will tell about new Nest. Instead of making things better they make it worst and harder. 

[Cheezer12]

Yup,  C wire was my issue as well.  2nd gen Nest worked fine without it but 4th gen seems to need it.

I put a multimeter measuring the 24V circuit and I noticed significant voltage drop every time thermostat calls for cooling.  I suspect that the voltage drop is causing the AC compressor contractor to open and AC compressor shuts off.  As soon as I hooked up the C wire, there is no voltage drop and AC compressor stays on.  Bottom line is Google should tell customer that there is a 99% chance they will need C wire hooked up.

[Cheezer12]

oops,  autocorrect got me.  it was supposed to say"AC compressor contactor switch". not contractor.

To add to my explanation I think the contactor switch coil is unable to keep the 240V circuit closed due to significant voltage drop (down to 14V to 16V for a second) in the 24V control circuit.

[RicardoF]

I have a C wire going to the thermostat and it still didn't work. An HVAC technician came to service the AC unit and that wasn't the issue. He couldn't figure out why it was working. His best guess was that I needed to upgrade to a 40VAC transformer. The tech specs for the thermostat say it requires a 20 to 30VAC transformer, so I should have been good to go. I'm not really interested in having to spend even more money just to upgrade from a thermostat that is working perfectly fine just because Google decided to stop supporting it.