Carrier 58MCB code 33: limit circuit fault is two different warnings
Read Carrier 58MCB code 33 as an open limit circuit, separate airflow checks from flame-rollout danger, and know why code 13 may follow.
What this means
Carrier 58MCB status code 33 means the limit circuit is open. That circuit includes the high-limit switch and flame-rollout switches, so the code does not tell you which safety opened. Check only the filter and unobstructed grilles; visible flame rollout, soot, a gas odor, or a returning code requires immediate service.
Equipment and model context
Carrier 58MCB Series 110 and 120 condensing gas furnaces
- 58MCB040
- 58MCB060
- 58MCB080
- 58MCB100
- 58MCB120
This interpretation is limited to the Carrier 58MCB Series 110 and 120 manual. Other Carrier furnace controls can use the same number with different timing or component groupings, so confirm the model on the furnace rating plate.
What you may notice
- The furnace burners stop, the blower continues, and the status LED repeats three short flashes followed by three long flashes.
- Code 33 changes to code 13 after the limit circuit remains open for the period allowed by the 58MCB control.
Likely causes
- A high-limit switch can open when furnace temperature rises above its allowed range, including when return or supply airflow is restricted.
- A flame-rollout switch can open because heat or flame is present where the burner compartment safety is designed to detect it.
Diagnostic paths
Burners stop but the blower keeps moving air
Heat begins, then the burners shut off before the thermostat is satisfied while the supply blower continues and code 33 appears.
Turn the thermostat off, inspect the accessible filter, and confirm return grilles and supply registers are not blocked. Do not keep cycling the furnace if the code returns.
There is heat, soot, or discoloration near the burners
You see soot, scorching, unusual burner-area heat, or flame movement outside the normal burner path through the factory viewing opening.
Switch the furnace off, leave it off, and arrange licensed gas-furnace service. Do not press a rollout reset or remove an access panel.
The display advances from 33 to 13
The initial 33 pattern is later replaced by code 13 while heating remains unavailable.
Report both codes in order. Code 13 records that the open limit circuit persisted; it is not a second component diagnosis.
Code 33 establishes an open safety circuit. The observed operating sequence and a technician's measurements are what separate an airflow-related high-limit opening from a combustion-area rollout opening.
- Code 33The series limit circuit is open
- High-limit branchAir-path temperature exceeded its allowed range
- Rollout branchHeat reached the burner-area safety
- Cause verifiedService tests the condition before replacing a part
| Observation | Supported reading | Owner action |
|---|---|---|
| Loaded or wrong filter | A visible airflow restriction exists | Replace with the specified clean filter |
| All grilles clear, code returns | The cause remains inside the system | Leave the furnace off for service |
| Soot, rollout, or gas odor | A combustion hazard may be present | Leave the area and follow gas emergency guidance |
| Code 13 follows code 33 | The limit circuit stayed open | Preserve both codes for the technician |
Code 33 names a circuit, not a failed switch
The 58MCB control watches a series safety circuit. Carrier's service instructions place the furnace limit and flame-rollout switches in that path, which is why one display number covers two conditions with sharply different risk. Ordering a limit switch from the code skips the first diagnostic question: which device opened, and was it responding correctly to heat?
The high-limit branch responds to temperature in the furnace air path. A loaded filter, obstructed return, closed supply path, blower problem, excessive temperature rise, or firing issue can all sit behind that event. The related high-limit switch definition explains why the blower can continue after the burners stop. That continued airflow is a cooling response, not proof that the blower or thermostat caused the trip.
The rollout half of the code changes the stop point
A flame-rollout device watches the burner area, not the supply-air temperature. If it opens, resetting it without establishing why heat reached that location defeats the warning. The shared code therefore has to be treated according to the higher-risk branch until a technician identifies which switch is open.
Your useful contribution is a clean record. Photograph the code through two repetitions, note whether the burners lit before it appeared, record whether the blower continued, and capture code 13 if it follows. Add the complete model and serial number using the HVAC data plate guide. Those details let service begin with the correct Carrier document instead of a brand-wide code chart.
What to give the technician
Provide the exact 58MCB model and serial number, code 33 video, any following code 13, filter size and condition, burner and blower sequence, and whether soot, odor, or unusual burner-area heat was present.
Safe homeowner checks
- With the furnace off, inspect only the normal homeowner-access filter and replace it with the specified size and type if loaded.
- Confirm return grilles and supply registers are open and clear without reaching into ducts or removing furnace panels.
- Record the full flash pattern, operating sequence, and any later code before power is interrupted.
Is a reset appropriate?
Do not repeatedly reset code 33. A returning open safety circuit needs its temperature or combustion cause identified before the furnace runs again.
When to call a technician
- Identifying the open device, measuring temperature rise, testing safeties, and inspecting combustion require licensed gas-furnace service.
- A rollout indication, soot, gas odor, or carbon-monoxide alarm is a stop condition, not a reset opportunity.
Questions people ask about this
What is the difference between Carrier codes 33 and 13?
Code 33 reports the limit circuit open. On the cited 58MCB control, code 13 follows when that circuit remains open beyond the allowed interval, so it records persistence rather than a new failed part.
Can a dirty filter cause Carrier code 33?
A loaded filter can restrict airflow enough to raise furnace temperature and open the high-limit branch. It cannot explain every code 33 because the same circuit also contains rollout safeties.
Should I press the flame-rollout reset?
No. A rollout trip requires combustion and heat-exchanger assessment. Pressing a manual reset without finding the cause removes the warning while leaving the hazard unresolved.
Evidence recordHow this page was checked
service manual, official manufacturer support article · checked 2026-09-09
Every technical claim above was written from primary documentation held in the HVAC Bench evidence record: Carrier technical literature. Where a source limits a definition to certain models, test conditions, or product classes, that limit is repeated here rather than generalised.
- Documentation class
- service manual, official manufacturer support article
- Scope of the definition
- Carrier 58MCB Series 110 and 120 condensing gas furnaces
- Last checked
- 2026-09-09