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Nest E74 no power to Rh wire. Tried troubleshooting

xstinaful
Community Member

‎02-28-2024 03:12 PM

Hand me down Nest gen 3 learning thermostat from previous owner. 

Have water source heat pump in a condo building with hydraulics not central air.

Worked fine for 2.5 years and then suddenly stopped charging and stopped working - showing E74 error code.

Already tried turning off power with breakers and safety switch on heat pump unit. 

Also reinserted wires, taken C wire out, switch Rh to Rc.

Nothing is working and all local techs I’ve called so far only work on central air.

 

Suggestions? Do i need to get a replacement? 

 

 

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2 REPLIES 2

RRS13
Community Member

‎02-29-2024 08:23 AM

Mine (also Nest Gen. 3) started doing the same exact thing. It's been powered fine, but all of a sudden it's getting a power failure error. I have power going to it, turned off/on the breaker, charged the thermostat overnight, reset the WiFi network setting, and still it's dying. My air handler hasn't even been on for months since I haven't needed it. I did cycle it a couple weeks ago and it was working fine. 

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Markjosephp
Community Specialist
Community Specialist
In response to RRS13

‎03-08-2024 10:59 AM

Hello folks,


I'm sorry to hear about the power issues you're facing with your Nest Thermostats. It sounds like you've already taken some troubleshooting steps. Let's sort this out.

 

  1. Make sure that your breakers are on.
  2. If you recently used your AC, check your system's drip pan or drain tubes.
    1. It might be full of water, or the drain tubes might have a clog. In these cases, the system might shut itself off to prevent an overflow of water.
  3. Check the condition of the HVAC fuse.
    1. Occasionally, fuses burn out due to one or more reasons. If the HVAC fuse burns out, the system might cut off the power to the thermostat and cause an E74 message code to appear.
  4. Check if you have an energy savings box.
    1. Some utility companies install boxes that cut off the power during high-demand (peak) times. These energy-saving boxes can cause errors.
  5. Check the air filter.
    1. A clog in the air filter can limit how much air flows over the cooling coils. If the clog persists, the cooling coils can freeze and cause the customer’s system to shut off.
  6. Gather the power readouts in the Technical Info with and without the C wire.
  7. Ensure that the thermostat wires are properly stripped and seated in the thermostat base.

 

Let us know how it goes.

Best regards,
Mark

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