Nest Thermostat “No Power to Rh Wire” (E74) and Other Power Errors

What the Error Means

The Nest detects a wire in the Rh (or Rc) connector but no 24 V on it. The thermostat itself is usually fine; power is not reaching it from your heating and cooling equipment. Google’s list of Nest wiring error codes groups them like this:

Note that E73 is Rc, not Rh — a common mix-up. Which code you have tells you which side of the system to look at.

Check These, in Order

1. Breakers and switches. Google’s first step: make sure the breaker or fuse for your system is on. Check the furnace or air-handler service switch (it looks like a light switch) and the outdoor unit’s breaker.
2. The furnace door. Most furnaces cut all power when the blower door is not fully seated. If anyone changed a filter recently, push the door until it latches.
3. The condensate float switch. Google notes that a float switch shuts the system down if water is not draining. If you see an Rc error in summer and there is water in the drain pan, this is the likely cause. Clear the drain line.
4. An overheating furnace. Furnaces have a safety switch that cuts power to part of the system when it gets too hot — often from a clogged filter. Replace the filter.
5. The fuse on the control board. Google says a blown fuse on the system’s control board can stop power reaching the thermostat. Power off first; it is usually a small 3 A blade fuse. A fuse that blows again means a short, which needs a technician.
6. The wires at the Nest. With the power off, Google specifies about 3/8 inch (6 mm) of bare wire in each connector and every connector button pressed down. Re-strip any that are short or corroded and re-seat them.

Our blank-screen guide walks through the same power path in more detail with a symptom table.

If the Error Comes and Goes

An intermittent Rh or Rc error, particularly on hot or cold days when the system runs hard, usually means a safety switch is tripping and resetting: the float switch in summer, the furnace high-limit in winter. Google has a separate alert, “Your A/C system briefly lost power several times”, for exactly this pattern.

A Nest that is short of power in a subtler way shows other signs: Google lists “Delayed” messages, reduced battery life and Wi-Fi drops as symptoms of low power. See Nest says Delayed.

The Permanent Fix for Low Power: Nest Power Connector or a C Wire

If everything above checks out and the Nest still struggles for power, give it a steady supply. Google’s Nest Power Connector works with the Nest Thermostat, Nest Thermostat E and the Nest Learning Thermostat (3rd and 4th gen), on 24 VAC systems only (not millivolt or high-voltage). The other option is a C wire, if there is a spare conductor in the cable.

Frequently Asked Questions

What does E74 mean on a Nest thermostat?

No power to the Rh wire detected. The Rh wire is connected, but the Nest is not seeing 24 V on it, which usually means the heating equipment has lost power: a breaker, the furnace door switch, a safety switch or a blown control-board fuse.

What is the difference between E73 and E74?

E73 is no power on the Rc wire (usually the cooling side). E74 is no power on the Rh wire (usually the heating side).

What does N72 mean on a Nest?

Google describes N72 as the Rh wire being detected but not connected to equipment. It is not a power loss.

Can a clogged AC drain cause a Nest power error?

Yes. A full drain pan trips the float switch, which cuts power to the system. Clearing the drain line restores it.

Related Guides

• Smart thermostat black or blank screen — the full power-path checklist
• Nest says Delayed — the low-power symptom
• Smart thermostat troubleshooting — every fix guide by symptom

Sources

• Google Nest Help: Nest thermostat wiring error codes
• Google Nest Help: Nest thermostat power and fuse issues
• Google Nest Help: system alerts
• Google Nest Help: signs of low power
• Google Nest Help: Nest Power Connector compatibility