Carrier Furnace Error Code 33: Limit Circuit Lockout Fix

Quick Answer

Carrier error code 33 means your furnace has overheated and locked out for safety. This is almost always caused by a dirty air filter or blocked vents. Replace your filter and ensure all registers are open to restore airflow and clear the code.

When your Carrier furnace flashes code 33, it's basically telling you it's suffocating. This code triggers when the high limit switch, a vital safety sensor, trips three times because the internal temperature got way too hot. I've been on these calls for 15 years and honestly, this is rarely a broken part. It's almost always a simple airflow restriction you can fix yourself without calling anyone.

CarrierFurnaceSeverity: highDifficulty: moderate92% DIY Success

Time to Fix

15–90 min

Difficulty

moderate

Parts Cost

$15 – $65

Tools Needed

Phillips #2 screwdriver, Nut driver set (1/4 inch and 5/16 inch)

What Does the 33 Code Mean?

Code 33 is a protective lockout, and I see it more than almost any other furnace code. Most of the time it's homeowners using those thick MERV 13+ allergen filters and forgetting to swap them out. The furnace gets too hot, the safety switch opens to prevent a fire, and after three attempts the board shuts the whole system down. Usually a $5 filter fix solves it.

Most Likely Causes

Based on aggregated repair data, here is the probability breakdown for this error code:

Dirty or restrictive air filter65%

Failing blower motor or capacitor15%

Blocked vents or return air10%

Faulty high limit switch7%

Dirty heat exchanger3%

Symptoms You May Notice

The furnace fires up, burners light, you get warm air for about 8-12 minutes, then everything shuts down and the status light starts blinking
Status LED flashes 3 times, pauses, repeats over and over
Air blasting out of the vents feels burning hot right before the system kicks off, way hotter than it should be
House won't get above 60°F even though the thermostat is set to 70°F and the furnace keeps cycling on and off
Blower motor sounds like it's straining, noticeably less airflow than usual through the registers

Can you reset a Carrier furnace to clear the 33 code?

Find the power switch on the side of the furnace or flip the dedicated breaker in your electrical panel to off. Wait a full 60 seconds so the control board can discharge completely. Flip it back on. If the limit switch has cooled down, the code should clear and the furnace will try to ignite. If it trips again within the first heating cycle, you've got an airflow issue that needs to be addressed before you reset it again.

Tools Required for Diagnosis

Phillips #2 screwdriverNut driver set (1/4 inch and 5/16 inch)Multimeter with continuity settingNon-contact voltage testerReplacement air filter (MERV 8 recommended, measure yours first)Flashlight or headlamp

Diagnostic Checklist

Follow these steps in order. We start with the easiest external fixes before opening up the machine.

ComponentComponent Under Test

Expected Range0–2 ohms

ConditionIf Open (OL) or infinite, replace component.

Replacement Parts

If your diagnostic testing proves the component has failed, you will need a replacement. We recommend OEM parts over aftermarket for water-handling components.

Part Name	OEM Number	Estimated Price
High Limit SwitchHH12ZB170 · $25–$65	HH12ZB170	$25 – $65
Blower Motor CapacitorP291-0503 · $15–$40	P291-0503	$15 – $40

Frequently Asked Questions

How do I reset my Carrier furnace after a code 33?

You can either wait about three hours for the control board to auto-reset, or cycle the power manually. Find the service switch on the side of the furnace or flip the dedicated breaker off and wait at least 60 seconds before turning it back on. That clears the board's temporary memory. But here's the thing, if the airflow issue isn't fixed first, the code's coming back within minutes of the next heating cycle. Don't just keep resetting it without figuring out why it's tripping.

Is it safe to run my furnace if it keeps showing code 33?

No, don't keep resetting and running it if code 33 keeps coming back. That limit switch is there to stop your heat exchanger from cracking or melting from the inside out. Ignore it long enough and you're risking a fire, or worse, a cracked heat exchanger that leaks carbon monoxide into your home. That's not me being dramatic, that's actually what happens. Think of code 33 like your car's oil pressure warning light. You wouldn't just tape over it and keep driving.

Can a bad thermostat cause a code 33?

Honestly, almost never. The thermostat just tells the furnace when to turn on or off, it's got nothing to do with temperature monitoring inside the heat exchanger. Code 33 comes from the furnace's own internal sensors. If your thermostat was the problem, you'd probably see the furnace not turning on at all, or short cycling without any error code showing on the board. I've literally never diagnosed a code 33 that traced back to the thermostat in 15 years of service calls.

Why does my furnace work for 10 minutes then show code 33?

Classic dirty filter or weak blower symptom. When the furnace first fires up, the heat exchanger is cold so it can handle restricted airflow for a few minutes. But after 8-10 minutes, heat builds up faster than the sluggish air can carry it away, and the limit switch trips. Same thing happens when a blower motor's starting to fail. It handles the load briefly but loses speed as the motor windings get hot. Pull your filter first. If it's more than 90 days old, that's almost certainly your answer.

Can I run the furnace without a filter to test if that's causing code 33?

Yeah, actually that's one of the first things I do on a service call to rule out the filter quickly. Pull it out and let the furnace run a full cycle without it. If it runs 20+ minutes without tripping code 33, your filter was the culprit. Just don't run it that way for more than a test cycle or two. Without a filter you're pulling dust straight into the blower wheel and heat exchanger, which creates a bigger mess that's way harder to clean up than just swapping a filter.

Related Carrier Furnace Error Codes

Code: 13Carrier Error 13: Limit Circuit Out of Range Code: 31Carrier Error 31: Pressure Switch Fault

Models Known to Experience 33 Errors

This repair applies to most Carrier furnaces with this error code. Common model numbers include:

58STA, 58WAV, 58PAV, 58MXA, 59SC5, 58CVA, 58CTX, 58TUA

Written by

Raj Patel

HVAC & Water Systems Specialist · 15 years experience

Last verified for technical accuracy on May 20, 2024