Nest Thermostat Warning W4 — Fix Guide

What Does This Error Mean?

Nest warning W4 means your Nest thermostat detected that your air conditioner is not cooling as efficiently as expected. The AC is running but the temperature is not dropping at the normal rate. This suggests a problem with your AC system that is reducing its cooling performance.

Affected Models

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

Common Causes

The air filter is clogged, restricting airflow through the AC system
The AC's evaporator coil is dirty or partially frozen, reducing cooling capacity
Low refrigerant is making the AC less effective at removing heat
The outdoor condenser coil is dirty and cannot release heat properly
The outdoor temperature is extremely hot, reducing the AC's efficiency

How to Fix It

Check and replace your air filter immediately. A clogged filter is the most common cause of reduced cooling. Replace it if it has been more than 1–3 months since the last change.

A blocked filter restricts how much air flows over the evaporator coil. Less airflow means less cooling. A clean filter is the fastest, cheapest fix.
Check the indoor air vents throughout your home. Make sure none are blocked by furniture, curtains, or other objects. Closed vents reduce airflow and make the AC work harder.

All vents in your home should be open and unobstructed for the AC to circulate air efficiently, even in rooms you do not use often.
Look at the indoor unit (air handler or furnace). Check the area around the evaporator coil for frost or ice. Ice formation indicates a refrigerant or airflow problem.

If you see ice, turn the AC off immediately and let it thaw completely (2–4 hours) before restarting. Call a technician to diagnose the root cause.
Clean the outdoor condenser unit. Turn off power at the breaker, remove any debris from around the unit, and gently rinse the condenser coils with a garden hose.

Dirty condenser coils cannot efficiently release heat from your home. Cleaning them annually improves efficiency and prevents W4 warnings.
If the W4 warning persists after filter and condenser cleaning, schedule an HVAC tune-up. A technician will check refrigerant levels, clean the evaporator coil, and measure cooling performance.

Annual AC tune-ups prevent efficiency problems and extend the life of the system. Addressing W4 now avoids a full breakdown later in the season.

When to Call a Professional

W4 is a warning — your AC is still running but underperforming. If changing the filter and cleaning accessible parts does not help, call an HVAC technician. Low refrigerant and dirty evaporator coils require professional service. Addressing efficiency issues promptly prevents more serious AC failures.

Frequently Asked Questions

How does Nest know my AC is underperforming?

Nest tracks how long it takes your home to cool by a certain amount. Over time, it learns your home's normal cooling rate. When the AC takes significantly longer than usual to reach the target temperature, Nest flags this as a potential efficiency problem and shows W4.

W4 only appears on very hot days. Is that normal?

Yes — on extremely hot days (above 95°F / 35°C), even a well-maintained AC can struggle to cool a home quickly. Nest may show W4 simply because the heat load is higher than usual, not because anything is wrong. If W4 only appears during extreme heat waves and not on normal summer days, your system may be working as designed.

Should I be worried about my AC because of W4?

W4 is a warning, not a critical error. Your AC is still running — it is just not performing at peak efficiency. Think of it as an early warning to service your system before a small problem becomes a big one. A simple filter change and condenser cleaning fix W4 in most cases.