Nest E74, E73 and E195 Errors: No Power to Rh, Rc or R Wire

1. What each Nest power code means

Google groups several help codes under the same message. The number varies with the model and software version; what matters is the connector named in the message. These are from Google’s Troubleshoot Nest thermostat help codes page.

E codes are critical: Google says heating or cooling is blocked until they are cleared. N codes are notices; the system may still run.

2. Why a Nest thermostat loses power on R

On a standard forced-air or heat pump system, the thermostat doesn’t take power from a wall outlet. A 24-volt transformer in the furnace or air handler sends power up the R wire, and the thermostat returns it on W, Y or G to start heat, cooling or the fan. Anything that stops the transformer, or breaks the R wire between the board and the wall, produces the “no power” code.

Rh and Rc are two names for that same power wire. Most homes have one transformer and one R wire; a few older systems with separate heating and cooling equipment have two transformers, one wire on each. Google’s jumper wire page says Nest Learning Thermostats join Rh and Rc internally, so a single R wire can go into either connector and no jumper is used. That is why the code names the connector your wire is in, not a wire the system is “missing”. See Rh and Rc explained and the two-transformer wiring setup.

Common causes, from most to least likely:

• The furnace or air handler is switched off at its service switch or breaker, or a separate heating or cooling switch is off.
• The R wire isn’t fully inserted in the Nest base, has too little bare copper, or is corroded.
• A small fuse on the HVAC control board has blown, often after a wire touched something during installation.
• The system shut itself off: a full condensate pan or drain, or an overheated furnace behind a clogged filter.
• The wire is broken or loose at the control board, not at the wall.

3. How to fix E74, E73 or E195, step by step

These steps follow Google’s “Troubleshoot power” steps and its thermostat won’t turn on page, with the meter check added. Turn power off at the breaker before touching any wire.

1. Check system power. Find the breaker or fuse for the furnace or air handler and make sure it is on, pushed all the way to On. Many furnaces also have a light-switch-style service switch on or near the unit. If your home has separate heating and cooling switches, both must be on. Then look at the thermostat again.
2. Close the furnace door properly. Most furnaces have a door safety switch that cuts power when the front panel is off or loose. If you opened the furnace during the install, refit the panel firmly.
3. Reseat the wires at the thermostat. With power off, pull the display off the base. Google says each wire should be fully inserted with its connector button down, with about 3/8 inch (around 1 cm) of clean, straight copper and no corrosion. Trim and re-strip a wire that looks dull, green or bent. Never put two wires in one Nest connector.
4. Check the connection at the control board. Open the furnace or air handler panel and follow the thermostat cable to the board. Make sure the R wire is tight on the board’s R terminal, and that the same color goes to the same function at both ends.
5. Check the control-board fuse. Google notes that a blown fuse on the system’s control board can stop power reaching the thermostat. Look for a small automotive-style blade fuse, usually 3 or 5 A, that is blackened or has a broken strip inside. If it is blown, find the short before replacing it: a common cause is a C wire (sometimes blue and labeled B) put on the wrong connector. Our B wire guide explains that mix-up.
6. Measure the voltage. With power on, set a multimeter to AC volts and measure between R and C at the wall, or at the board. About 24 to 28 V AC means power is reaching the wire. No reading with a good fuse points to the transformer, a safety switch or a broken wire. See how to test thermostat wires with a multimeter.
7. Restore power and wait. Put the display back, turn the breaker on and give the thermostat a few minutes. If the screen stays dark on a model with a USB port, Google suggests charging it over USB for at least 10 minutes.

If the code clears, check the wiring screen in the Google Home app (Settings, then Thermostat, then Wiring) matches the wires actually installed.

4. When the code appears in hot weather

Google has extra steps for E3, E4, E23, E24, E73, E74 and E195 that show up in summer, and says you will likely need an HVAC professional. Before calling, check:

• The air filter. A dirty filter restricts airflow and can freeze the cooling coil. Replace it if it is grey or clogged.
• The drip pan and drain tubes under the indoor coil. Water in the pan or a clogged drain can cause the system to shut off; many units have a float switch that does this on purpose to prevent a leak.
• The HVAC fuse for blackening, discoloration or a broken filament.
• The breakers, which may have been switched off during work on the house.

If clearing the drain makes the code go away, the problem may come back until the drain is cleaned properly or the coil is serviced.

5. Codes right after installing a Nest

If the old thermostat worked and the code appeared the moment you switched on the new one, the likely causes are installation ones:

• A blown fuse from a bare wire touching another wire, the metal box or the furnace cabinet while power was on.
• The furnace door switch left open, or the service switch left off.
• A jumper wire from the old thermostat left in place. Remove it; Nest Learning Thermostats don’t use one.
• The R wire in the wrong connector, for example in C or a W connector. Check every label against your photo of the old wiring.

If you still have the old thermostat, reinstalling it is a quick test: if it doesn’t work either, the fault is on the HVAC side. Unsure which old label goes where? The wiring tool maps each terminal onto the Nest connectors, and the Nest Learning Thermostat (4th gen) wiring guide covers that model. If your problem is that the Nest keeps losing power without an R code, read the no C wire guide.

6. When to call an HVAC technician

• A replacement fuse blows again.
• There is no 24 V between R and C at the control board with the fuse good and every switch on.
• The furnace control board shows a fault light or code you can’t clear.
• The cooling coil is frozen or the drain keeps backing up.
• You smell gas or burning, or a breaker trips.

For other screen messages and their causes, see thermostat display messages explained, and for general faults the thermostat troubleshooting checklist.

7. Frequently asked questions