Error codes · Carrier

Carrier Furnace Code 33 — Limit Switch Open

Code 33 on a Carrier (or Bryant/Payne) furnace means the high-limit circuit opened — the heat exchanger got hotter than allowed, almost always from restricted airflow. Fix the airflow first; only replace the limit if it fails after airflow is restored.

Updated July 2026 · 6 min read · Canada

Symptoms

Furnace starts, then shuts off mid-cycle
Status LED flashes code 33 / limit switch open
Supply registers feel weak or rooms stay cold

Likely causes

Dirty or undersized air filter
Closed supply registers or blocked returns
Failing blower motor / capacitor
Stuck or weak high-limit switch (less common)

What to do

1. Power down and check the filter

Turn the thermostat to Off, then switch the furnace switch off. Pull the filter and hold it to a light — if you cannot see light through it, replace it with the exact size stamped on the frame.

2. Open airflow paths

Open at least 80% of supply registers, clear furniture from returns, and vacuum the blower compartment intake if accessible. Restricted airflow is the #1 cause of code 33.

3. Reset and watch one full heat cycle

Restore power and call for heat. If the furnace completes a full cycle without tripping, the filter/airflow fix was enough. If code 33 returns immediately with a clean filter, escalate.

4. Only then consider the limit switch

A limit that opens with free airflow may be weak. Matching the OEM part number is safer than a universal substitute. Capacitor issues on the blower can also starve airflow.

Stop and call a pro when

You smell burning plastic or see scorched wiring
The furnace is a sealed combustion unit and you cannot safely access the blower
Code 33 returns after a new filter and open registers
You are uncomfortable working near gas appliances

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Carrier Code 13 — Carrier/Bryant code 13 is a limit lockout after repeated trips. Fix airflow first, then check filters or capacitors.
Goodman 4 Flashes — What Goodman/Amana 4-flash limit codes mean, how to clear airflow restrictions safely, and which filter sizes or blower parts usually stop the trips.
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FAQ

Is Carrier code 33 dangerous?

The limit switch is a safety device. Do not bypass it. Fix airflow or call a technician — running with a jumpered limit risks overheating the heat exchanger.

Will a higher MERV filter make code 33 worse?

Yes, if the system was already marginal. If you recently jumped from MERV-8 to MERV-13 and code 33 started, try MERV-11 or a deeper media cabinet.

Need a licensed tech instead?

If the guide says to stop — or the part swap did not clear the fault — get free quotes from licensed local HVAC contractors.

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