Nest Thermostat Error E8 — What It Means & How to Fix It

What it means

E8 means the Nest detected a heat pump fault.
Heat pumps both heat and cool by reversing the refrigerant flow, so E8 covers either side of that — wrong O/B wire setup, a stuck reversing valve, a compressor that won't start, or a frozen-over outdoor unit.
The fix is usually a settings tweak in the Nest app or an HVAC visit.
It's not a Nest hardware fault.

Affected Models

Nest Learning Thermostat (1st, 2nd, 3rd gen)
Nest Thermostat E
Google Nest Thermostat (2020)

Common Causes

O/B wire (the reversing valve wire) is configured wrong for your heat pump brand — Nest sees opposite-of-expected behaviour
Reversing valve itself failed and won't switch modes
Compressor not starting — bad capacitor, low refrigerant, or mechanical fault
A pressure safety switch tripped and locked the heat pump out
Outdoor unit iced over and the defrost cycle isn't clearing it

How to Fix It

Check the O/B wire setting in the Nest app first.

Settings > Equipment > Continue > find the O/B wire screen.
Most brands (Carrier, Trane, Lennox) energize O/B in Cooling.
Rheem, Ruud, and some Yorks energize in Heating.
Wrong setting means the Nest swaps your heat and cool calls — that alone is enough to trigger E8.
Power off the HVAC, pull the Nest off, and reseat the O/B wire.

A loose O/B wire interrupts the reversing valve signal and confuses the Nest about which mode the system is actually in.
Push the wire fully into the terminal and snap the Nest back on.
Check whether the outdoor unit is iced over.

Some frost on the coil in winter is normal.
A unit completely encased in ice is not operating.
Switch to emergency heat to take load off the heat pump and let it defrost — if the defrost cycle isn't clearing ice on its own, that's a technician job.
Check the outdoor unit's breaker and disconnect.

Outdoor units run on a dedicated circuit.
A tripped breaker or blown disconnect fuse kills the whole heat pump and the Nest reads it as a fault.
Reset once — if it trips again immediately, stop and call a technician.
Call HVAC if E8 persists.

Tell them whether the system produces nothing, or produces the opposite of what you ask for.
That distinction tells the tech where to start — power-side fault vs reversing-valve fault.
They'll check refrigerant pressures, compressor amps, and reversing valve operation.

When to Call a Professional

If it's not the O/B setting (which is a five-second app fix) you need an HVAC technician.
Heat pumps have more moving parts than basic AC, and reversing-valve diagnosis, refrigerant checks, and compressor work all need proper tools.
Ask for a technician with heat pump experience — not all HVAC techs are equally fluent.

Frequently Asked Questions

What is a heat pump and how is it different from an air conditioner?

An AC only cools — it moves heat out of the house.
A heat pump moves heat in both directions: out in summer, in from outside in winter.
The reversing valve is what flips the direction.
They're efficient in mild climates but lose efficiency below about -1°C (30°F), which is why they pair with backup electric or gas heat (called auxiliary or emergency heat).

My heat pump heats in summer and cools in winter — is that E8?

Classic O/B wire issue.
The Nest is energizing the reversing valve at the wrong moment because it has the wrong assumption about your heat pump brand.
Settings > Equipment > flip the O/B setting between 'Energized in Cooling' and 'Energized in Heating'.
Fix is instant.

Can a Nest run a heat pump without a C-wire?

Most heat pump installs already have a C-wire even if it's not connected — check the air handler.
The Nest needs constant power to avoid battery-drain communication errors with the system, and a C-wire is the cleanest way to give it that.
If there's no C-wire to be found, the Nest Power Connector adds one without running a new cable.