Google Nest Thermostat Error E38 — What It Means & How to Fix It

What Does This Error Mean?

Nest error E38 means the thermostat has detected a condition that triggered a safety lockout. The Nest has stopped allowing the HVAC system to run because it detected something unsafe — such as the system running continuously without reaching the target temperature, a heat pump running in cooling mode when it should be heating, or an auxiliary heat system that failed to activate. E38 is a protective shutdown to prevent equipment damage or dangerous operating conditions. Do not try to bypass this error — find and fix the underlying issue first.

Affected Models

Nest Learning Thermostat (3rd gen)
Nest Learning Thermostat (4th gen)
Nest Thermostat E

Common Causes

The HVAC system ran for an extended period without reaching the thermostat's set temperature, triggering an over-run safety lockout
A heat pump operating in an incorrect mode — such as trying to heat with the cooling circuit — was detected and locked out
Auxiliary or emergency heat failed to activate when the heat pump needed backup, and the Nest locked out to prevent equipment damage
A wiring error is sending conflicting signals that make the Nest detect a safety condition
The HVAC system has an actual fault (frozen coil, failed compressor, or refrigerant issue) that is preventing it from conditioning air

How to Fix It

Note the exact message shown with E38. Nest sometimes shows additional context such as 'System running too long,' 'Heat not reaching target,' or similar. This message helps identify the specific safety condition that was triggered.

On the Nest display, tap the error code to see more detail. In the Nest app, open the device card and check the Alerts section for a more detailed explanation.
Reset the safety lockout. Go to Settings > Reset > Schedule or force a restart by going to Settings > Reset > Restart. Alternatively, turn off the Nest's system at the thermostat (set to Off), wait 5 minutes, and turn it back on.

This clears the lockout and lets the system try again. Watch it carefully for the next hour to see if it is actually reaching your set temperature.
Check the air filter in your HVAC system. A severely clogged filter restricts airflow so badly that the system cannot reach the set temperature. Replace or clean the filter if it has not been done in the last 1-3 months.

A dirty filter is the most common reason an HVAC system runs and runs without reaching the target temperature. Replacing it takes 5 minutes and often resolves E38 immediately.
Check the Nest's heat pump settings. Go to Settings > Equipment and review the system type configuration. Ensure the Nest is configured correctly for your system — whether it is a heat pump, conventional, or dual-fuel system.

A Nest configured for the wrong system type will operate the equipment in a way that triggers safety lockouts. If the system type was never correctly set up, reconfiguring it can resolve E38.
Inspect the outdoor unit if you have a heat pump. Look for a frozen coil or blocked airflow around the unit. If the outdoor coil is completely encased in ice, turn the system to emergency heat and call an HVAC technician.

A heat pump that cannot exchange heat properly will fail to reach the target temperature. The Nest detects this extended failure and locks out the system to prevent compressor damage.

When to Call a Professional

Error E38 often signals a real problem with the HVAC system itself, not just the thermostat. If the system is not heating or cooling to the set temperature even after a reset, the HVAC system needs inspection. A failing compressor, frozen coil, or refrigerant issue requires a licensed technician. Do not keep resetting the Nest repeatedly hoping E38 will go away — the repeated lockouts exist to protect your equipment.

Frequently Asked Questions

Is it safe to keep resetting E38 to get heat back on?

Resetting it once to test if the problem has resolved itself is fine. But if E38 keeps coming back after each reset, the underlying problem has not been fixed. Continuously resetting the lockout while the root cause persists can damage your compressor. Compressors are the most expensive component in an HVAC system — a replacement can cost $1,500 to $3,000 or more. Address the root cause rather than repeatedly overriding the safety lockout.

What does it mean when the Nest says the system is running too long?

The Nest monitors how long the HVAC system runs and compares it to how quickly the temperature is actually changing. If the system runs for a very long time without the temperature moving toward the setpoint, the Nest concludes something is wrong. It could be a clogged filter, a refrigerant leak, an iced-over coil, an outdoor unit that is not running, or simply very extreme outdoor temperatures. The Nest locks out to prevent the system from running indefinitely with no result.

How do I check if my auxiliary heat is working?

Set your thermostat to a temperature well above the current room temperature — say, 10 degrees higher than the current temperature. In very cold weather, the heat pump alone may struggle, and the auxiliary heat should kick in automatically. You can confirm auxiliary heat is running in the Nest app — it will show 'Aux' or 'Emergency Heat' on the display when it is active. If auxiliary heat never activates even in very cold weather, the auxiliary heat circuit has a fault that needs a technician to diagnose.