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

What it means

Your HVAC is running, but the room temperature isn't changing the way the Nest expects.
That's the difference from E32 — the system did turn on; it's just not getting anywhere.
Most common cause is a clogged filter.
Then: closed vents, low refrigerant, a dead heat strip, or extreme outdoor temperatures.

Affected Models

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

Common Causes

Air filter severely clogged — system runs but airflow is choked
AC low on refrigerant from a leak — runs but barely cools
Heating element in an electric furnace or heat pump aux died
Vents closed or blocked, conditioned air not reaching the Nest sensor
Outdoor temperature beyond what the system can overcome — running flat-out but not catching up

How to Fix It

Replace the air filter. Right now.

First and most important step.
It's in the return vent or inside the furnace cabinet.
A severely clogged filter is the single most common cause of E33 — a $10 filter swap clears the error in a meaningful share of cases.
Walk the house and check the vents.

Every supply vent should be open and not blocked by furniture or rugs.
The big return vent (where air gets pulled back to the system) needs to be clear too.
If the room with the Nest sensor isn't getting conditioned air, the Nest will never see the temperature move.
Check whether interior doors are blocking airflow.

Two-story homes especially: if the Nest is upstairs and the air handler is on the main floor, closed bedroom doors can isolate the Nest's room from the rest of the house.
Crack the doors and see if it helps.
If you're cooling, look at the outdoor unit.

Fan should be spinning.
No ice on the coil or the refrigerant lines.
Ice means low refrigerant or restricted airflow — turn cooling off, run fan-only for 2–3 hours to thaw, replace the filter, then try again.
If the ice comes back, you need refrigerant service.
If you're heating below 35°F outdoor, the heat pump may just be out of capacity.

Heat pumps lose efficiency fast as outdoor temperatures drop.
Below freezing, the system may simply not be able to heat the house alone — switch to Emergency Heat (electric strips) until conditions ease.
Recurring E33 in cold weather usually means aux heat isn't kicking in when it should.

When to Call a Professional

If a filter change and an open-vents check don't fix it, call an HVAC technician.
Refrigerant leaks, dead heat strips, and a heat pump struggling in extreme cold all need a tech with gauges and a meter.
An annual tune-up — filter, refrigerant, airflow — usually catches E33 before it becomes a service call.

Frequently Asked Questions

What's the difference between E32 and E33?

E32: the system never turned on — wiring, a fuse, or a safety switch.
E33: the system did turn on and is running — but it's not reaching the set temperature.
Different problems, different parts of the system.

Can weather alone trigger E33 without anything broken?

Yes.
Above ~100°F or below ~10°F, even a healthy system may not reach a typical set point.
The Nest's internal model says 'the temperature should have moved by now' and triggers E33 because reality doesn't match.
If it only happens during extreme weather and otherwise works fine, you're at the equipment's design limits — adjust the set point on those days.

How does the Nest know the temperature isn't changing enough?

It builds a learned model of your specific HVAC over weeks — degrees per hour under different conditions, how the house responds to heat and cold.
When the system runs much longer than the model expects without reaching the set point, that's E33.
The learned model is smarter than a dumb thermostat — but it also means E33 can show up briefly after a real performance change (for example, right after a filter change improves airflow).