Carrier Furnace Fault Codes: What Each LED Pattern Means

Carrier furnaces signal faults through a blinking LED on the control board, visible through the sight glass on the furnace door or with the lower panel removed. A handful of codes mean the same thing across most Carrier control board generations. Past those, the code-to-fault map genuinely changes by board, which is why the sticker inside your furnace’s door panel, not a generic chart, is the definitive source for your unit.

TL;DR: Code 33 (limit circuit fault, usually a dirty filter) and Code 13 (limit lockout, a safety cutoff) are the two most common Carrier codes and mean the same thing on virtually every Carrier control board. Beyond the codes below, check your furnace’s own door sticker before assuming a code’s meaning: Carrier has used multiple control board families with different numbering for setup errors and motor-calibration faults.

Bryant and Payne furnaces built on the same control board family as your Carrier unit use identical codes. All three are Carrier Global Corporation brands sharing hardware, but “same brand family” doesn’t mean “same board” across every model year. Match by control board, not just badge.

First step always: open the lower door panel and find the sticker. Every Carrier furnace has a fault code chart on the inside of the door panel, specific to your control board. That chart is the definitive source; use this guide to understand the context and confirm what to check first, not as a replacement for it.

How to read a Carrier furnace fault code

1. Locate the LED. With the lower door panel open or while watching through the sight glass, find the status LED on the control board, typically a red or amber light near the center of the board.
2. Count the short flashes. The first group is short flashes: this is the tens digit.
3. Count the long flashes. After a pause, the second group is long flashes: this is the ones digit.
4. Combine them. Three short flashes + three long flashes = Code 33. Two short + four long = Code 24.
5. Watch at least two full cycles to confirm the count before acting on it.

If your LED doesn’t show a clear short-flash/long-flash split, your board may use a different display convention. Check the door sticker for the exact pattern format your control board uses.

Carrier furnace fault codes: verified consistent across board generations

These five codes carry the same fault name on both a Carrier 58MVP-family control board and a 58STA-family control board, two genuinely different control platforms, which is the closest thing to a universal Carrier code list that actually holds up.

Codes 21-23, 31-32, and 41-45 are not on this table on purpose. Carrier’s own manuals disagree with each other here: on a 58MVP-family (two-stage/variable-speed) control board, 21-23 are setup/model-selection errors and 41-44 are blower- and inducer-speed calibration faults, not the gas-valve or motor-failure meanings some guides publish for those same numbers. If your furnace shows a code in that range, match it against your door sticker or your model’s specific service manual rather than a generic chart. Publishing a wrong meaning here is exactly how someone ends up chasing the wrong repair.

Code 33: Limit circuit fault (the most common Carrier furnace code)

Code 33 means the high-limit switch or a rollout switch has opened. These are safety devices that interrupt the burner circuit when the furnace overheats or flames extend outside the burner box.

In most Code 33 cases, the fix is a clogged air filter. When airflow is restricted, the heat exchanger can’t transfer heat fast enough. Plenum temperature rises above the high-limit switch setpoint (roughly 170-200°F depending on model), and the switch opens. Replace the filter, wait for the furnace to cool, then reset.

Step-by-step for Code 33:

1. Check the filter first. Pull the filter (usually in the return air duct or a slot on the side of the furnace). Hold it up to light. If you can’t see light through it, replace it.
2. Check airflow throughout the system. Walk the house: every supply register open, the main return grille unblocked, no collapsed flex ducts.
3. Reset the furnace. After filter replacement and any airflow fixes, turn power off for 30 seconds, restore, and call for heat. It should complete a normal cycle without retriggering Code 33.
4. If Code 33 returns with a clean filter and clear airflow: the blower motor may be slowing or failing (listen for labored sound or slow spin-up), the high-limit switch itself may have failed stuck open (a tech can confirm with a meter), or, if trips keep recurring after filter and airflow are confirmed good, a cracked heat exchanger is worth ruling out before the next heating season.

Safety box: if Code 33 is accompanied by Code 13 (limit lockout after repeated trips), stop using the furnace and call a licensed technician. Repeated limit trips indicate a persistent overheating condition, and resetting the furnace over and over instead of finding the cause is how a cracked heat exchanger goes undetected. A cracked heat exchanger can let combustion gases into your air stream; this is not something to work around with resets.

Code 13: Limit circuit lockout

The high-limit or rollout switch opened enough times (typically more than 10) in one heating cycle that the board locked out rather than keep cycling. This is a persistent-overheating signal, not an isolated event. Work through the Code 33 steps above (filter, airflow), and if the lockout keeps returning, call a licensed HVAC tech before running the furnace further.

Code 14: Ignition lockout

The furnace attempted ignition three consecutive times without confirming a flame. The board entered lockout.

Reset: turn the furnace power off for 30 seconds, restore, and call for heat. Watch what happens on the next attempt.

What to look for:

• No glow visible through the sight glass: igniter failed or not receiving power. See Furnace Hot-Surface Igniter: Symptoms and Replacement.
• Igniter glows but no flame: gas not reaching burners. Check the gas valve’s manual lever (should be open) and confirm gas service to the house is on. If gas supply is confirmed, the issue may be the gas valve or pressure switch; call a tech.
• Flame appears briefly then shuts off: that’s Code 34 (ignition proving failure), covered next.

Code 34: Ignition proving failure (dirty flame sensor)

The igniter lit the gas, but the flame sensor couldn’t confirm the flame, most likely because the sensor rod is coated in oxide. Result: the furnace lights for 5-10 seconds, shuts off, retries, and locks out.

Clean the flame sensor. It’s a thin metal rod inside the burner box with a ceramic mounting block and a single wire. Turn off power, remove the sensor (one screw), clean the rod with 0000 steel wool or fine emery cloth, reinstall. See Furnace Flame Sensor: Symptoms and Cleaning.

Code 24: Secondary voltage fuse blown

The 3A fuse on the control board, which protects the 24V low-voltage circuit, has blown, usually from a short in the thermostat wiring.

Check:

• Look for the small fuse on the control board (usually labeled “3A FUSE” or similar). A blown fuse is visually evident: the wire inside is broken.
• Replace it with a fuse of the same amperage. If it blows again immediately, there’s a short somewhere in the thermostat wiring. Look for a wire touching a metal casing, staple-damaged wire, or a shorted thermostat.

When to call a licensed HVAC technician

• Code 13 (limit lockout): stop running the furnace until diagnosed.
• Code 33 returns after filter replacement and airflow check: possible heat exchanger inspection needed.
• A code in the 21-23, 31-32, or 41-45 range that you can’t match on your door sticker: these vary by control board, and a wrong guess risks the wrong repair.
• Any code that traces back to the gas valve, inducer motor, blower motor, or control board itself.
• If the furnace has any visible scorch marks, or you smell gas or burning at any point: turn off the furnace and gas supply and call immediately.

FAQ

Are Bryant furnace codes the same as Carrier? Only if the two units share the same control board family. Bryant and Carrier are both Carrier Global Corporation brands and often use identical boards, but “identical boards” isn’t guaranteed across every model year. Check the door sticker on your specific unit rather than assuming.

My Carrier furnace shows a fault code but then runs fine. Should I worry? The code shows the most recent fault, not necessarily an active one. If the furnace runs normally, the fault may have been intermittent. But if Code 33 or Code 14 appears even occasionally, it’s worth checking the filter and flame sensor rather than ignoring it.

How do I clear a fault code on a Carrier furnace? Turn the power switch off for 30 seconds and restore. This resets the board and clears the stored code. If the underlying fault is still present, the code returns on the next heating cycle.

Can I see fault code history on a Carrier furnace? Most Carrier residential boards only display the most recent fault. Carrier’s Infinity communicating systems with an Infinity Touch thermostat display history in the thermostat’s diagnostic menu.

My Carrier furnace shows Code 33 in summer when it shouldn’t be running. If the AC is running and the air handler shares the furnace cabinet, the limit switches can still trip from a dirty air coil restricting airflow. Same fix: check the filter and evaporator coil.

Related guides

• Carrier HVAC: Brand Overview, Reliability, and Repair Help: product lines, Infinity vs. standard tiers, serial number age lookup, and warranty details for all Carrier equipment.
• Furnace Not Heating: Diagnostic Order: if no fault code is showing and the furnace simply isn’t heating.
• Furnace Flame Sensor: Symptoms and Cleaning: Code 34 leads here.
• Furnace Hot-Surface Igniter: Symptoms and Replacement: Code 14 lockout with no glow through the sight glass leads here.

If a fault code is pointing toward system replacement, our HVAC brands guide explains the rebadge map (Carrier is also Bryant and Payne) and what to look for when comparing bids.

Last reviewed August 2026. Fault codes vary by control board generation, so always confirm against your furnace’s own door sticker.