Nest Thermostat Error E33 — What It Means & How to Fix It

What Does This Error Mean?

Nest error E33 means the HVAC system is running, but the temperature in your home is not changing the way the Nest expects. The thermostat confirmed the system turned on (unlike E32), but after running for a while, the temperature has not moved toward the set point. This tells you the system is running but not heating or cooling effectively. Common causes include a very dirty air filter, a refrigerant leak, a failed heating element, or the outdoor temperature being extreme.

Affected Models

Nest Learning Thermostat (all generations)
Nest Thermostat E
Nest Thermostat (2020 model)
Nest Learning Thermostat (4th Gen)

Common Causes

The air filter is severely clogged, allowing the system to run but blocking enough airflow to prevent effective heating or cooling
The air conditioner is low on refrigerant due to a leak, so it runs but cannot cool the air properly
A heating element in an electric furnace or heat pump has failed, so heat is not being generated despite the system running
All vents in the home are closed or blocked, so conditioned air cannot circulate to where the Nest sensor is located
The outdoor temperature is so extreme that the HVAC system simply cannot overcome the heat load — the system runs constantly but cannot reach the set temperature

How to Fix It

Replace the air filter immediately. This is the first and most important step. Locate the filter in the return air vent or inside the furnace cabinet and replace it if it is dirty.

A severely clogged filter allows the system to run but blocks so much airflow that almost no heat or cool air reaches the living space. A $5 to $20 filter replacement is the fastest potential fix for E33.
Walk through your home and check that all supply vents are open and unobstructed. Also check that the return air vent (the large vent that pulls air back to the system) is not blocked.

Closed or blocked vents reduce airflow throughout the system. If the Nest sensor is in a room that is not receiving conditioned air, it will see no temperature change and report E33.
Check that doors between rooms are reasonably open. Closed interior doors can prevent conditioned air from reaching the area where the Nest is installed.

This is especially relevant in two-story homes. If the Nest is upstairs but the heating or cooling source is primarily on the main floor, closed doors can prevent the system from reaching the upstairs sensor.
For cooling issues, inspect the outdoor condenser unit. Make sure it is running (the fan should be spinning) and that the coils are not coated in ice. Ice on the coils indicates low refrigerant or an airflow problem.

If you see ice on the refrigerant lines or the coil, turn off cooling and run the fan only for 2 to 3 hours to thaw the ice. Then replace the filter and try cooling again. If ice returns, the system is low on refrigerant.
For heating issues in very cold weather, check if your heat pump is struggling. If outdoor temperatures are below 35°F, a heat pump loses efficiency rapidly. If you have emergency heat, enable it temporarily.

Heat pumps work by moving heat from outside air into your home. When outdoor temperatures drop below freezing, there is less heat to extract and the system struggles. Emergency heat (usually electric resistance strips) takes over when the heat pump cannot keep up.

When to Call a Professional

If replacing the air filter and checking vents does not resolve E33, call an HVAC technician. Low refrigerant, a failed heating element, and a struggling heat pump in extreme cold all require professional diagnosis. An HVAC tune-up — including refrigerant check, filter replacement, and airflow testing — is the most comprehensive solution for a recurring E33 error.

Frequently Asked Questions

What is the difference between E32 and E33?

E32 means the HVAC system did not turn on after the Nest sent the command — the system is not running at all. E33 means the system did turn on and is running — but the temperature is not changing toward the set point. E32 is usually a wiring, fuse, or safety switch problem. E33 is usually an airflow, refrigerant, or heating element problem. Both need attention, but they point to different parts of the system.

Can extreme weather cause E33 without anything being broken?

Yes. On very hot summer days (above 100°F) or very cold winter days (below 10°F), even a healthy HVAC system may struggle to reach the set temperature. The E33 error triggers when the Nest's learned model says 'the system should have reached this temperature by now' but has not. If E33 only appears during extreme weather and the system otherwise works fine, the HVAC equipment is likely at its design limits rather than broken. Adjusting the set temperature to something less extreme on such days prevents the error.

How does Nest know the temperature is not changing enough?

The Nest learns how quickly your specific HVAC system typically changes the home temperature over time. It builds a model based on weeks of observation — how many degrees per hour your system heats or cools under different conditions. When the system runs for significantly longer than the learned model predicts without reaching the set temperature, the Nest triggers E33. This learned model makes the Nest smarter than a basic thermostat but also means E33 can sometimes appear after the system's performance changes — like after a filter change restores airflow.