Carrier furnace limit switch
Verify the OEM part number and temperature rating before replacing a confirmed failed limit switch.
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Error Code: Carrier 13
What it means: Code 13 is a hard lockout triggered by repeated limit circuit faults. The furnace control board detected high temperatures inside the heat exchanger section, recorded multiple limit switch trips within a single call for heat, and stopped running to protect the system.
This is different from code 33. Code 33 (Limit Circuit Fault) is a warning. It tells you the limit switch has opened once, but the furnace reset itself and kept running. Code 13 is the hard lockout that happens after three consecutive code 33 events or one sustained limit trip beyond the safety timer. Think of code 33 as the yellow light and code 13 as the red light with the engine shut off.
How the Limit Circuit Works
Carrier furnaces use one or more limit switches mounted on or near the heat exchanger. Each switch is a normally-closed device. When the air temperature around the switch rises above its rated opening temperature (typically 140-200 degrees Fahrenheit depending on the model), the switch opens. This interrupts the 24-volt control circuit and closes the gas valve.
The blower continues running. That is by design. The blower cools the heat exchanger down so the limit switch can reset. If the switch cycles open and closed three times in one call for heat, the board locks out with code 13.
Code 13 vs Code 33
| Code | Meaning | Action Required |
|---|---|---|
| 33 | Limit circuit fault (warning) | Check airflow, furnace may self-recover |
| 13 | Limit circuit lockout (hard stop) | Root cause must be fixed before reset holds |
If you catch code 33 on a Carrier furnace, you have time to address the airflow issue before it escalates to code 13. If the furnace is already showing code 13, the limit switch has already opened enough times to trigger the lockout.
Common Causes Ranked by Frequency
1. Dirty or Wrong Air Filter
The most common cause by a wide margin. A filter that is more than 90 days old in a normal home can restrict airflow enough to trigger limit trips. A MERV 13 or higher filter on a furnace that requires MERV 8 can also cause overheating. Check the filter first and replace it with the correct size and rating.
2. Blocked or Closed Supply Registers
Too many closed registers create backpressure that reduces airflow through the heat exchanger. Walk the house and confirm every register is open. Furniture blocking a return register is also common. The return side needs unobstructed airflow to feed the blower.
3. Undersized or Dirty Blower Wheel
A blower wheel caked with dust and debris moves less air. Remove the blower compartment panel and inspect the wheel. A layer of dirt on the blades can reduce airflow by 30-40 percent. Clean the wheel with a stiff brush and shop vacuum.
4. Failing Blower Motor
A blower motor with a weak run capacitor, failing bearings, or ECM module problems runs at reduced speed. The heat exchanger overheats because the blower cannot move enough air to carry the heat away. Check the run capacitor with a multimeter set to capacitance mode. The reading should match the label value within 5-10 percent.
5. Dirty Evaporator Coil (A-Coil)
On systems where the evaporator coil sits above the furnace, a dirty A-Coil restricts airflow on the supply side. This is a common cause after a summer cooling season. The coil collects dust that blocks air passages. Cleaning the coil requires access to the plenum.
6. Cracked Heat Exchanger
A cracked heat exchanger can cause hot spots and local overheating even when the blower is running correctly. This is less common than airflow issues but far more serious. A cracked heat exchanger allows carbon monoxide to enter the airstream and requires full replacement.
7. Failed Limit Switch
Limit switches themselves can fail. After hundreds of thermal cycles, the bimetallic disc can fatigue and open at a lower temperature than intended. This is rare but worth testing if all other causes have been ruled out.
Diagnostic Workflow
Step 1: Read the Code and Clear It
Look at the LED on the control board. If it is flashing a sequence, count the flashes. On Carrier SureLight boards, the diagnostic LED shows a two-digit code followed by a pause, then the code repeats. Code 13 appears as a single flash followed by three flashes (1, 3). Write down the code. Power off the furnace at the breaker for 30 seconds. Restore power and run a heat cycle. If code 13 reappears immediately or within minutes, proceed.
Step 2: Check Temperature Rise
Measure the return air temperature at the filter grille or the return plenum. Measure the supply air temperature as close to the heat exchanger outlet as possible. Subtract the return temperature from the supply temperature. The difference is the temperature rise. Carrier furnaces typically target a 40-70 degree Fahrenheit rise. If your measurement is above 80 degrees, airflow is the problem.
Step 3: Check the Filter
Pull the filter. Hold it up to a light. If you cannot see through it, replace it. Verify the filter is the correct size. Check the MERV rating. MERV 8 is standard for most Carrier residential furnaces. MERV 13 filters are popular for allergy season but can restrict airflow enough to cause limit trips during cold weather when the furnace runs longer cycles.
Step 4: Test the Limit Switch
With the furnace off and cool, locate the limit switch on the heat exchanger. Disconnect the two wires. Set your multimeter to continuity or resistance mode. Touch the probes to the two switch terminals. A good limit switch shows near zero ohms (closed circuit) when cool. An open reading tells you the switch is either tripped and needs time to cool, or it has failed open.
Step 5: Check the Rollout Switch
Carrier furnaces also have rollout switches mounted near the burner manifold. These are manual-reset safety devices. If the rollout switch has tripped (button popped out), do not push it back in without understanding why. A tripped rollout switch means the flame is rolling out of the burner tube instead of staying inside the heat exchanger. This is a serious safety issue. Call a technician.
Parts Table
| Part | Typical Cost | Notes |
|---|---|---|
| Limit switch (Carrier OEM) | $15-$40 | Part number varies by model |
| Blower capacitor (match specs) | $10-$30 | Replace if reading outside 5% of rated |
| Air filter (MERV 8) | $8-$25 | Replace every 90 days |
| ECM blower motor module | $200-$500 | Carrier OEM required |
| Rollout switch (manual reset) | $15-$35 | Small red button on switch body |
Resetting Code 13
Turn the thermostat to OFF. Flip the furnace breaker to OFF. Wait for the control board LED to go completely dark. This takes roughly 30 seconds. Flip the breaker back ON. Set the thermostat to call for heat. Watch the furnace through one full cycle. If code 13 returns within that first heat cycle, the root cause has not been addressed. Do not continue resetting without diagnosing the problem.
Bryant and Payne Note
Bryant and Payne furnaces share the same control boards as Carrier. Code 13 means the same thing on all three brands. The diagnostic workflow is identical. If you have a Bryant or Payne showing code 13, follow this guide.
When To Call a Professional
If you have checked the filter, opened all registers, verified the blower runs at full speed, and code 13 still appears, stop running the furnace and call a technician. The remaining possibilities include a cracked heat exchanger, a blocked secondary heat exchanger (on 90%+ models), or a failed blower motor that needs replacement. All three require professional tools.
A cracked heat exchanger can leak carbon monoxide into your home. If you or anyone in the home has symptoms like headaches, nausea, or dizziness when the furnace runs, leave the house and call your gas utility from outside.