Carrier Furnace Error 13 indicates a Limit Circuit Lockout. This occurs when the furnace’s primary or auxiliary limit switches have opened several times within a single heating cycle or remained open for longer than three minutes. Effectively, the control board has detected a dangerous overheating condition and has disabled the ignition sequence to prevent permanent damage to the heat exchanger or a potential fire hazard.
⚠️ Safety First: Read Before Repairing
Incorrect repairs can cause fire or injury. Always verify with the manufacturer’s manual.
If you are experiencing this, you likely notice your blower motor running continuously while only cold air blows from your vents, or perhaps a total lack of response from the thermostat. It can be frustrating to be left in the cold, but don’t worry—this is a protective safety feature. In many cases, it is a solvable issue involving airflow restoration or a simple component replacement that you can handle with the right guidance.
Symptoms
Identifying Carrier Error 13 is straightforward if you know what to look for. The most obvious sign is the LED diagnostic light on the furnace control board (visible through the sight glass); it will flash 1 short pulse followed by 3 long pulses. Physically, you will likely notice the blower fan running non-stop. This is the furnace’s “cool down” mode attempting to flush heat out of the cabinet. Furthermore, the burners will not ignite, meaning your home temperature will steadily drop despite the thermostat being set to “Heat.” You might also hear the furnace “short cycling”—trying to start, clicking, and then immediately shutting down the gas valve.
How to Fix Carrier Error 13 (Step-by-Step)
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Perform a Power Reset:
Before diving into the hardware, try a hard reset. Flip the furnace power switch (or the circuit breaker) to the “Off” position for at least 60 seconds. This clears the lockout memory on the control board. If the furnace fires up and stays on, the issue may have been a temporary fluke, but you should still proceed with the following inspections. -
Inspect and Replace the Air Filter:
Safety Warning: Always turn off the power before opening the furnace cabinet.
Slide out your air filter. If you cannot see light through it when holding it up to a lamp, it is restricted. Replace it with a new filter. Avoid “ultra-restrictive” high-MERV filters if your system isn’t designed for them, as they can actually cause Error 13 by restricting air too much even when clean. -
Check Vents and Returns:
Walk through your home and ensure that at least 80% of your supply registers are open. Closing too many vents to “save energy” actually causes heat to back up into the furnace. Also, ensure no furniture or heavy drapes are blocking the large return air intakes. -
Test the Limit Switch for Continuity:
Locate the primary limit switch (usually a small rectangular piece with two wires, mounted on the central heat exchanger partition). Remove the two wires. Set your multimeter to Ohms (Ω) or Continuity mode. Touch the probes to the two terminals on the switch. If the furnace is cool, the switch should be “Closed” (show near 0 Ohms or beep). If it shows “Open” (OL) while the furnace is cold, the switch is defective and must be replaced. -
Inspect the Blower Wheel:
Check the blower compartment for debris. If the fan blades (squirrel cage) are coated in a thick layer of dust, they cannot “bite” the air effectively. Use a soft brush and vacuum to clean the blades. While there, check the capacitor for leaks or bulging, which could indicate the fan isn’t reaching full speed.
- Difficulty: Intermediate
- Estimated Time: 45 – 90 Minutes
- Tools Needed: Phillips head screwdriver or 1/4″ nut driver, Multimeter (for continuity testing), Replacement high-efficiency air filter, Soft-bristle brush or vacuum.
- Estimated Cost: $0 (Cleaning/Filter) to $150 (Professional Limit Switch Replacement).
What Triggers this Code?
Understanding the “why” is crucial for a permanent fix. Error 13 is rarely a random glitch; it is usually a response to a mechanical or environmental failure within the system.
- Restricted Airflow (The Most Common Cause): If air cannot move across the heat exchanger, the heat builds up until it exceeds the safety threshold of the limit switch. This is often caused by a heavily clogged air filter or obstructed return air grilles. Over time, dust buildup acts as an insulator, driving temperatures to critical levels.
- Blower Motor Failure: If the blower motor is failing or the capacitor is weak, the fan may not spin at the required RPMs. Without sufficient cubic feet per minute (CFM) of air moving through the system, the heat exchanger overheats almost instantly.
- Faulty Limit Switch: These switches use a bi-metal disc designed to warp and break the circuit at a specific temperature. Over years of “thermal cycling” (expanding and contracting), the metal can become fatigued or “drift” in its calibration, causing it to trip at much lower, safer temperatures than intended.
- Dirty Evaporator Coil: If your furnace is paired with an A/C unit, the evaporator coil sits right above the heat exchanger. If this coil is choked with pet hair or dust, it acts like a wall, trapping heat inside the furnace cabinet and triggering the lockout.
How to Prevent Error 13
Consistency is the enemy of furnace breakdowns. To ensure your limit circuit stays closed and your home stays warm, follow these preventative measures:
- Strict Filter Schedule: Change 1-inch filters every 30-90 days. If you have pets or live in a dusty area, check them monthly. This is the single most effective way to prevent overheating.
- Annual Static Pressure Check: During your yearly professional tune-up, ask the technician to check the “Total External Static Pressure.” This measures how hard the fan has to work to push air. High pressure indicates ductwork issues that lead directly to Error 13.
- Clean the AC Coil: If you use central air, have your evaporator coil cleaned every 3-5 years. A clean coil ensures that heat can escape the furnace unimpeded during the winter months.
Frequently Asked Questions
Q: Can I bypass the limit switch to get heat temporarily?
A: No. Never bypass a safety limit. These switches prevent the heat exchanger from cracking and leaking Carbon Monoxide into your home or causing a fire. If the switch is tripping, it is doing its job to protect your life and property.
Q: Why does the code return even after I replaced the filter?
A: The lockout might need a manual reset, or there is a secondary restriction. Check the “Rollout Switch” (usually located near the burners with a small red button in the middle). If the furnace overheated severely, this manual-reset switch may have popped out and needs to be pushed back in.
Q: How much does a replacement limit switch cost?
A: The part itself is relatively inexpensive, usually ranging from $15 to $50. However, ensure you match the temperature rating stamped on the side of the old switch (e.g., L200-40F) to ensure the new one operates within the manufacturer’s safety specs.