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

What Does This Error Mean?

Nest error E8 means a fault was detected in the heat pump system. Heat pumps provide both heating and cooling by reversing the refrigerant cycle — and E8 indicates the Nest detected that the heat pump is not operating as expected in either mode. This can be a wiring issue with the O/B reversing valve wire, a heat pump system fault, or a compressor problem. This error requires professional attention in most cases.

Affected Models

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

Common Causes

The O/B wire (reversing valve wire) is wired incorrectly for the specific heat pump brand, causing the Nest to switch heating and cooling modes backwards
The reversing valve itself has failed and cannot switch the heat pump between heating and cooling mode
The heat pump compressor is not starting due to a failed capacitor, low refrigerant, or a mechanical fault
A refrigerant pressure safety switch has tripped and locked out the heat pump
The outdoor unit is iced over in winter and the defrost cycle is failing, preventing proper heat pump operation

How to Fix It

Check the O/B wire configuration in the Nest app. Open the Nest app, go to Settings > Equipment > Continue. On the screen showing your wiring, find the O/B wire setting. There is an option to set the O/B wire to energize in Cooling or in Heating mode — this depends on your heat pump brand. Most brands (Carrier, Trane, Lennox, Rheem) energize O/B in Cooling. Some brands (Rheem, Ruud, some York) energize in Heating.

An incorrect O/B setting makes the heat pump run in the opposite mode to what you want — calling for heat produces cool air and vice versa. If your system was recently installed or if the Nest is new, verify this setting against your heat pump's manual.
Turn off HVAC power and check the O/B wire connection at the Nest base. Pull the Nest off the base and ensure the O/B wire is fully seated in the O/B terminal. An improperly connected reversing valve wire causes the heat pump to fail to change modes, triggering E8.

The O/B wire controls the reversing valve, which physically switches the heat pump between heating and cooling mode. Even a momentary loss of connection at this wire can disrupt heat pump operation.
Check whether the outdoor unit is iced over. Look at the outdoor unit — in winter, some frost on the coil fins is normal, but a unit that is completely encased in ice cannot operate. If iced, turn the system off and let it defrost naturally, or set the heat pump to run in emergency heat mode while the outdoor unit defrosts.

Heat pumps defrost themselves automatically in normal operation. If ice builds up and does not clear, the defrost cycle is malfunctioning — which is an HVAC technician repair.
Check the outdoor heat pump unit's circuit breaker and disconnect box. The outdoor unit has a dedicated circuit. A tripped breaker or blown disconnect fuse cuts power to the entire heat pump — both heating and cooling — causing E8.

Reset a tripped breaker by switching it fully off and then back on. Do not repeatedly reset a breaker that keeps tripping — a repeatedly tripping breaker indicates an electrical fault that needs professional investigation.
Call an HVAC technician experienced with heat pumps. Describe whether the system produces neither heat nor cold, or produces the opposite of what you want. The technician will check refrigerant pressures, compressor operation, reversing valve function, and control board signals.

Heat pump diagnosis is more involved than standard AC work. When calling, specifically ask for a technician with heat pump experience — not all HVAC technicians are equally familiar with heat pump systems.

When to Call a Professional

E8 on a heat pump almost always requires an HVAC technician. Heat pump systems are more complex than simple AC systems. Reversing valve diagnosis, refrigerant system checks, and compressor testing all require professional tools. Call a technician who specifically works with heat pump systems.

Frequently Asked Questions

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

An air conditioner only cools — it moves heat from inside your home to outside. A heat pump does both heating and cooling — it moves heat from outside into your home in winter (heating mode) and from inside to outside in summer (cooling mode). It does this by reversing the refrigerant cycle using the reversing valve. Heat pumps are very efficient in mild climates. In very cold weather (below about 30°F / -1°C), they need backup electric or gas heat — called auxiliary or emergency heat.

My heat pump heats in summer and cools in winter. Is that E8?'

This is classic backwards O/B wire configuration in the Nest settings. The Nest is reversing the heating and cooling calls because it does not know whether your heat pump energizes the O/B wire in heating or cooling mode. Go to Settings > Equipment in the Nest app and change the O/B wire setting from 'Energized in Cooling' to 'Energized in Heating' (or vice versa). This immediately fixes a heat pump that runs backwards.

Can a heat pump system work with the Nest without a C-wire?

Heat pump systems usually have more wires than simple AC systems, which means there is more likely to be a C-wire available. Check the wiring at your air handler — there is often an unused C-wire terminal even in older systems. A C-wire ensures the Nest always has power and avoids battery drain issues that can cause communication errors. If no C-wire is available, the Nest Power Connector accessory provides an alternative power path without requiring a new wire.