Nest E73: no power to the Rc wire, and the cooling side behind it
E3, E23, and E73 all report no power at Rc. Google lists breakers, filters, condensate drainage, fuses, and wiring checks; the code does not name the failed part.
What this means
These codes report that the thermostat is no longer receiving 24 volt power on a named wire. Google groups E3, E23, and E73 as no power to the Rc wire, E4, E24, and E74 as no power to the Rh wire, and E195 as no power to the R wire, so the letter in the message identifies which transformer feed went quiet rather than which component failed.
Equipment and model context
Google Nest Learning Thermostat and Nest Thermostat
- Google Nest Learning Thermostat
- Google Nest Thermostat
- Google Nest Thermostat E
This page is written around the Rc group of Nest help codes. HVAC Bench covers the Rh group, headed by E74, on its own page, and the two should not be read across: on a two-transformer installation they point at different equipment in different parts of the building. Whether your system has separate Rc and Rh feeds at all is a property of the installed equipment.
What you may notice
- The thermostat shows a help code naming the Rc, Rh, or R wire and stops controlling the system.
- The display goes dark or disconnects from the app after the code has been present for a while.
- Cooling or heating stops working during the season when the affected side of the system runs hardest.
Likely causes
- Google attributes these codes to the heating and cooling system failing and no longer powering the thermostat.
- Google names a clogged air filter, water backing up in the drip pan and drain tubes, and a blown or blackened system fuse among the conditions to check.
- Google notes that wires can be crossed at installation, with the Rc and Rh conductors swapped in their connectors.
| Code | What Google says it means | Where to look first |
|---|---|---|
| E3, E23, E73 | No power to the Rc wire | The cooling side: outdoor unit, condensate switch, cooling transformer |
| E4, E24, E74 | No power to the Rh wire | The heating side: furnace or air handler power and its switch |
| E195 | No power to the R wire | A single-transformer system with one power feed |
| E298, E448 | No system power at all | Breakers and the equipment service switch |
| M20 | No power alert | The same supply questions, reported as a message |
| Code appears after new wiring | A connection rather than a failure | Rc and Rh may have been crossed |
Diagnostic paths
The code names Rc and it is cooling season
The message reports no power to the Rc wire, and it appeared during hot weather while the air conditioning was running.
Google's own framing for these codes is a cooling system that has broken down and stopped powering the thermostat. Check the air filter and look for water in the drip pan, because a condensate backup that shuts the equipment down removes the supply the thermostat was drawing from.
The code names Rh and the heating has stopped
The message reports no power to the Rh wire and the furnace or air handler is not responding.
Check the service switch beside the heating equipment and the breaker feeding it. A switch left off after maintenance removes the heating transformer supply, which is what the Rh conductor carries.
The code appeared right after an installation
The thermostat was recently replaced or rewired, and the code has been present since.
Turn off power to the heating and cooling equipment before touching thermostat wiring. Google then asks you to confirm that Rc and Rh were not crossed during installation and to correct them if they were, and to check each wire is straight, clean, and fully inserted.
The battery warning follows the power code
The power code remains and the thermostat later reports a low battery or drops off the app.
Google notes that a persistent power problem drains the internal battery and the thermostat may disconnect. Treat the battery message as a consequence of the original code rather than as a new fault.
What Rc and Rh actually are
Rc and Rh are power inputs, not outputs. Rc carries 24 volts from the cooling transformer and Rh carries 24 volts from the heating transformer. Systems built around a single transformer bring one wire, labelled R, and bridge the two inputs internally or with a jumper.
Because these are inputs, a code naming one of them is a statement about the equipment upstream rather than about the thermostat. The thermostat has detected that a supply it expects is absent, which is a measurement rather than a diagnosis.
That is why the code letter is worth reading carefully. On a two-transformer installation, Rc going quiet points at the cooling equipment and Rh going quiet points at the heating equipment, and those are different pieces of hardware in different parts of the building.
Why a condensate problem shows up as a thermostat error
Google's troubleshooting sequence includes checking the drip pan and drain tubes for a backup. A condensate safety switch can be wired to interrupt a low-voltage control or power conductor when water rises, so the thermostat may report the missing Rc supply even though the transformer itself remains energised.
Google lists the air filter and the drainage check as separate steps. A dirty filter may contribute to cooling-system trouble, while water in the pan may operate a safety switch; the E73 code does not prove a causal chain between the two.
Water under the indoor unit therefore makes the drain and its safety wiring relevant, but clearing drainage restores thermostat power only when that safety circuit is what opened it.
What the code does not prove
It does not prove the thermostat is faulty. Nothing in Google's published cause list points at the display, the base, or the internal battery as the origin, and the battery warning that follows is described as a consequence of the power loss rather than its source.
It does not identify which component upstream failed. A blown fuse, an open safety switch, a tripped breaker, and a failed transformer all present as an absent 24 volt supply, and the code cannot separate them.
It also does not confirm the wiring is correct. Google's instruction to check whether Rc and Rh were crossed exists because a code can report a genuinely absent supply on a terminal that should never have had a wire in it.
The checks worth making before calling
Start with the breakers and the service switch beside the furnace or air handler, because Google puts those checks first and they require no equipment panel to be opened.
Then look at the filter and at the area under the indoor unit. Both are on Google's list, both are owner-level, and both remove a cause rather than merely observing it.
Turn off power to both the heating and cooling equipment before the wiring inspection Google describes: each conductor straight, at least a centimetre of clean copper exposed, free of corrosion and paint, and pushed in far enough that the connector button stays down. Beyond that, a missing 24 volt supply means opening equipment panels, which is where owner work ends.
The thermostat measures the presence of a supply that originates in the equipment. Each stage of that path can remove it, and the code names only the terminal where the absence was noticed.
- Breaker and service switchThe first two places a supply is lost
- Equipment transformerProduces the 24 volts that Rc or Rh carries
- Safety devicesA condensate switch may interrupt a control or power conductor
- Rc, Rh, or R terminalWhere the thermostat notices the supply is gone
What to give the technician
Give the exact code, whether it names Rc, Rh, or R, when it first appeared and what the weather was doing, whether the filter was dirty, whether any water was present under the indoor unit, and whether the thermostat had recently been installed or rewired. Add the make and model of the furnace or air handler and the outdoor unit.
Safe homeowner checks
- Check the breakers for the heating and the cooling equipment and the service switch beside the furnace or air handler.
- Replace the air filter if it is dirty, which Google includes in its own troubleshooting sequence.
- Look for water in the drip pan or a blocked drain tube under the indoor unit.
- Turn off power to both the heating and cooling equipment before touching any thermostat wire.
- With power off, confirm each thermostat wire is straight, clean, and inserted until the connector button stays pressed down.
- With power off, check whether Rc and Rh were crossed if the thermostat was recently installed, and correct them if so.
Is a reset appropriate?
There is no reset that clears these codes while the supply is missing. The message goes away when 24 volts returns to the named terminal, so restoring the breaker, the switch, or the drainage is the reset. If the internal battery has drained, allow charging time after power returns before judging whether the fault has cleared.
When to call a technician
- A blown or blackened system fuse, whose cause needs to be found before replacement.
- A condensate blockage that needs the drain line cleared.
- An absent 24 volt supply once breakers, switches, and wiring have been confirmed correct.
- Any equipment panel that would need to be opened to reach a transformer or a safety device.
- Repeated recurrence of the same code after the drainage or filter has been corrected.
Questions people ask about this
What is the difference between E73 and E74 on a Nest?
They name different wires. Google documents E3, E23, and E73 as no power to the Rc wire and E4, E24, and E74 as no power to the Rh wire, so one points at the cooling side of a two-transformer system and the other at the heating side.
Does E73 mean my Nest thermostat is broken?
Nothing in Google's published cause list points at the thermostat itself. The codes report a 24 volt supply that is no longer arriving, and Google attributes that to the heating and cooling system rather than to the control.
Why does Google tell me to check the air filter for a wiring error?
Because E73 reports missing power rather than proving a wiring error. Google lists the filter and the drip pan as separate cooling-system checks; a condensate safety switch may interrupt the thermostat supply, but its wiring varies by installation.
What does E195 mean on a Nest thermostat?
Google documents E195 as no power detected on the R wire. It appears on installations with a single power feed rather than separate Rc and Rh conductors, so there is one supply to investigate rather than two.
Can I fix a no power to Rh wire error myself?
Some of it. Breakers, the equipment service switch, the air filter, the drip pan, and the seating of each wire are all owner-level checks Google publishes. A blown fuse or a missing transformer output means opening equipment panels, which is not.
Why did my thermostat also report a low battery?
Google notes that when the power problem persists the internal battery drains and the thermostat may disconnect. The battery message follows from the original power code rather than being a separate fault.
Evidence recordHow this page was checked
official manufacturer support article · checked 2026-09-08
Every technical claim above was written from primary documentation held in the HVAC Bench evidence record: Google Nest and Trane technical literature. Where a source limits a definition to certain models, test conditions, or product classes, that limit is repeated here rather than generalised.
- Documentation class
- official manufacturer support article
- Scope of the definition
- Google Nest Learning Thermostat and Nest Thermostat
- Last checked
- 2026-09-08