Nest Thermostat Error E4: Why Is There No Power to Rh and How Do I Fix It?
- No 24V reaching the Rh (heating) wire
- Tripped furnace breaker or service switch off
- Furnace door safety switch left open after service
Problem Description
Nest error E4 means no power is detected on the Rh wire — the wire that carries 24V to your heating side. Without Rh power the Nest cannot run your furnace or boiler and may lose power itself. E4 usually means the heating side has lost power: a tripped furnace breaker, an open furnace door safety switch, a blown control-board fuse, or a loose Rh wire.
Why This Happens in Real Homes
E4 is Google's code for "no power to the Rh wire" — the heating side's 24V feed. In real homes the number-one cause is embarrassingly simple: the furnace door panel wasn't clicked shut after a filter change, so its safety switch cut all power. Blown control-board fuses and, on high-efficiency furnaces, a tripped condensate float switch are close behind.
Start at the furnace — confirm the breaker and service switch are on, seat the door panel firmly, and check the fuse and drain — then reseat the Rh wire at both ends. Meter 24-28V across Rh and C to separate a wiring break from an upstream power problem.
Symptoms
- Error E4 on the thermostat
- Heating will not turn on
- Thermostat loses power or drops offline
- Furnace does not respond to a heat call
- Cooling may still work
- Battery low in Technical Info
- Error after furnace service
- No 24V at the Rh terminal
Recognize these? Here's what usually causes it.
Common Causes
- No 24V reaching the Rh (heating) wire
- Tripped furnace breaker or service switch off
- Furnace door safety switch left open after service
- Blown low-voltage fuse on the furnace board
- Condensate float switch tripped on a high-efficiency furnace
- Loose or corroded Rh wire at base or board
- Missing Rc-Rh jumper on a single-transformer system
- Failing heating-side transformer
Most fixes happen in the first 3 steps.
Do not bypass E4 error by jumping wires. Improper wiring can damage furnace or create safety hazards.
Tools & Requirements
These tools will help you complete this fix.
Multimeter
Klein Tools 80196 Digital Multimeter Kit with Case, ...
Screwdriver
STREBITO 155 in 1 Electric Screwdriver Set, Small El...
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Step-by-Step Solution
Understand E4
E4 means the Nest detects no 24V power on the Rh wire, which feeds the heating side. The thermostat cannot run the furnace and may lose power. The cause is almost always upstream at the furnace — lost power, an open safety switch, or a blown fuse — not the thermostat itself.
Check furnace power
At the furnace, confirm the circuit breaker is on and the service switch (a light-switch-style toggle on or near the unit) was not left off after maintenance. Look for a status LED on the control board; if it is dark, no power reaches the board or the Rh wire.
Check the door switch and fuse
Make sure the furnace access panel is fully closed — the door safety switch cuts all power if it is ajar, a very common cause after a filter change. Then inspect the 3-5A blade fuse on the control board and replace if blown. On high-efficiency furnaces a full condensate pan can trip a float switch that cuts power, so clear the drain.
Reseat the Rh wire
Switch off power. Pull the Nest and confirm the Rh wire has about 1cm of clean copper, sits fully in the Rh terminal, and lands firmly on the R/Rh terminal at the furnace board. On single-transformer systems, confirm the Rc-Rh jumper is in place.
Meter voltage or call a pro
With power on, meter across Rh and C at the board: 24-28V AC is normal. No voltage means a tripped safety, blown fuse, or failing transformer upstream; correct voltage with E4 still showing means a break in the Rh wire in the wall. Persistent E4 needs an HVAC technician.
Quick Solutions
Still having issues? This is usually the deeper cause below.
If drain continues after replacing batteries, check the event history — a stuck-open sensor or rapid polling loop burns through batteries in days.
If you have a heat pump, the heating wire might be connected differently. Check your system type in Nest settings.
Thermostat issues that keep returning are often caused by stale backup-battery memory holding old settings across power cycles without the user realising.
- No 24V reaching the Rh (heating) wire
- Tripped furnace breaker or service switch off
- Furnace door safety switch left open after service
- Blown low-voltage fuse on the furnace board
- Condensate float switch tripped on a high-efficiency furnace
Before you go — try one of these (they fix most cases).
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Official Manufacturer Manual
Google Nest provides official product documentation through their online manual rather than downloadable PDF. Access setup guides, troubleshooting steps, and product specifications for your Google Nest Thermostat.
Source: google.com
Need More Help? Google Nest Support
Note: The contact information below connects you directly to Google Nest's official customer support team, not Trunetto. They can help with warranty claims, device replacements, and advanced technical issues.
How Does Google Nest Compare?
Before replacing your Google Nest device, see how it stacks up against alternatives in our full comparison guides.
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Guide Improvements
- Updated July 4, 2026
Corrected the E4 wire mapping — the guide described "no power to W1," but Google's official meaning is no power to the Rh (heating) wire.
What changed:- Corrected the wire from W1 to Rh (heating) per Google's official help codes
- Updated the title and problem description to the heating-side power diagnosis
- Rebuilt causes and steps around the furnace breaker, the door safety switch left open after a filter change, the control-board fuse, and the Rc-Rh jumper
- Added a 24-28V meter check across Rh and C
Source: Editorial Accuracy Review
