Nest Thermostat Error E30 — What It Means & How to Fix It

What Does This Error Mean?

Nest error E30 means the thermostat detected an unexpected or incorrect signal on the O/B wire — the wire that controls whether your heat pump is in heating or cooling mode. Heat pumps work differently from regular AC or furnace systems: they use a reversing valve to switch between heating and cooling. The O/B wire tells the valve which direction to go. If Nest detects that this wire is not behaving as expected, it shows E30 and stops the system to prevent running the heat pump in the wrong mode.

Affected Models

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

Common Causes

The O/B wire setting in the Nest configuration is incorrect — set to O when it should be B, or vice versa, depending on the heat pump brand
The O/B wire is loose or partially disconnected from the Nest terminal
The reversing valve in the heat pump has failed or is stuck in one position
The O/B wire was placed in the wrong terminal during Nest installation
A short circuit or wire contact issue is causing incorrect voltage on the O/B terminal

How to Fix It

Check the O/B wire configuration in the Nest app or on the thermostat. Go to Settings > Equipment > Continue through setup > and look for the O/B wire setting. Make sure it is set correctly for your brand of heat pump.

Trane, American Standard, and most other brands use O (energized in cooling mode). Rheem and Ruud heat pumps typically use B (energized in heating mode). Setting this incorrectly is the most common cause of E30.
Turn off the HVAC breaker, pull the Nest off its base, and check the O/B terminal. Press the orange connector button, pull the wire out, and reinsert it firmly until seated, then release the button.

Even a slightly loose O/B wire can cause intermittent signal problems that trigger E30. This is a simple fix that takes less than a minute.
Check for any stray wire strands that may be touching adjacent terminals in the Nest base. Trim any copper strands with scissors and reinsert each wire cleanly into its own terminal.

A stray wire strand bridging the O/B terminal to the Y or G terminal can cause the reversing valve circuit to behave abnormally and trigger E30.
Restore power and test both heating and cooling modes. Set the thermostat to heat mode and raise the temperature above room temperature. Listen for the outdoor unit to start. Then test cooling the same way.

If the system heats when you set it to cool, or vice versa, the O/B setting (O vs B) is definitely configured the wrong way. Change it in the Nest equipment settings.
If the O/B setting is correct, the wiring is clean, and E30 persists, activate emergency heat (if available) to stay comfortable and call an HVAC technician to inspect the reversing valve.

Emergency heat bypasses the heat pump and uses a backup heating element or gas furnace. It is less efficient but keeps your home warm while the heat pump issue is diagnosed.

When to Call a Professional

If fixing the O/B setting and reseating the wire does not resolve E30, call an HVAC technician who is experienced with heat pumps. A stuck or failed reversing valve requires refrigerant system diagnosis and potentially valve replacement — this is a job for a licensed professional. Reverting to emergency heat (if your system has it) will keep you warm while you wait for service.

Frequently Asked Questions

What is the O/B wire and what is a reversing valve?

A heat pump works by moving heat rather than creating it — it moves heat from outside into your home in winter, and from inside to outside in summer. A reversing valve inside the outdoor unit physically switches the direction of refrigerant flow to switch between heating and cooling modes. The O/B wire is the signal wire that tells the reversing valve which way to go. If this signal is wrong, your heat pump could run in heating mode when you want cooling, or not switch modes at all.

How do I know if my heat pump uses O or B?

Most heat pumps made by Carrier, Lennox, Trane, American Standard, Goodman, and most other major brands use the O setting (the reversing valve is energized in cooling mode). Rheem and Ruud heat pumps are the main exception — they use B (energized in heating mode). Check your heat pump's outdoor unit label or the equipment documentation. You can also look at your old thermostat — if the O/B wire was in the O terminal, use O. If it was in B, use B.

Is it safe to use emergency heat while E30 is showing?

Yes — emergency heat is specifically designed for situations where the heat pump itself cannot operate. It uses a backup heating system (usually electric resistance heating strips or a gas furnace) to heat your home. It is safe to use but costs more to run than the heat pump, so use it only as a temporary measure. Set the thermostat to Emergency Heat mode under Settings > Equipment or directly on the thermostat display.