Carrier Furnace Error Code 13: Limit Switch Lockout Fixes and Troubleshooting

Meta Description: Carrier furnace code 13 means a limit switch lockout caused by overheating or airflow issues. Learn safe DIY checks, professional diagnostics, costs, and how to reset and prevent future lockouts.

Seeing Carrier furnace code 13 can be alarming. This error points to a limit switch lockout—your furnace is overheating and shutting down to protect itself. The good news: many causes are fixable, especially airflow problems. This guide explains what code 13 means, safe steps to diagnose it, when to call a pro, and how to prevent it from returning.

What Carrier Furnace Code 13 Means

Carrier furnace code 13 = Limit switch lockout. The control board detects repeated trips of the high‑limit switch during a call for heat and locks the furnace out to prevent damage. Most Carrier, Bryant, and Payne models auto‑reset after about three hours, or on power cycle, once the heat exchanger cools.

The high‑limit switch is a temperature safety device mounted on or near the heat exchanger or supply plenum. If supply air is too hot—usually from restricted airflow or overfiring—it opens, shutting burners off. If this happens several times in one heating cycle, the board posts code 13 and locks out.

Typical root causes include dirty filters, blocked return or supply ducts, a clogged evaporator (A‑coil), a slow or failed blower, incorrect blower speed, high external static pressure, or improper gas input. Less commonly, wiring faults or a bad limit switch are to blame.

Common Symptoms You Might Notice

• Burners light, run a few minutes, shut off early; blower keeps running to cool the furnace.
• Multiple short heating cycles followed by no heat and Carrier furnace code 13 on the control board LED.
• Some or many supply registers feel weak or barely warm.
• Furnace feels hot to the touch around the cabinet or supply plenum.
• After a power cycle, heat returns briefly, then the issue repeats.

Safety First

Do not bypass or jumper a limit or flame rollout switch. These are life‑safety devices. Bypassing can cause fire or heat exchanger damage.

Turn off power at the switch or breaker before removing panels. Gas adjustments, internal wiring changes, and heat exchanger inspection are professional‑only tasks.

Quick Checks Homeowners Can Do

These simple checks address the most common airflow restrictions that trigger code 13. Many homeowners can resolve the issue without tools.

• Air filter: Replace a dirty or high‑resistance 1‑inch filter. Use a quality MERV 8–11 in 2–4 inch format when possible. Ensure the arrow points toward the furnace.
• Vents and returns: Open at least 80% of supply registers. Clear furniture, rugs, and drapes. Unblock return grilles and replace clogged return filters.
• Evaporator coil access: If the A‑coil sits above the furnace, remove the top panel and visually check for matted dust on the downstream (supply) face. If dirty, schedule a professional coil cleaning.
• Blower compartment: Inspect for heavy dust on the blower wheel vanes; excessive buildup warrants professional cleaning.
• Thermostat fan setting: Set to Auto. Ensure Heat mode is selected and temperature is a few degrees above room temperature.
• Doors and panels: Replace access panels firmly. Many furnaces will not run properly if the blower door switch is not depressed.

How To Read The Code On A Carrier Furnace

Open the lower furnace panel to view the control board LED. Carrier boards flash a two‑digit code separated by a pause. Count the flashes to confirm error code 13. Some models also store last faults; consult the service sticker or manual for the exact legend.

Step‑By‑Step Troubleshooting Guide

Start with airflow. If the limit is opening, it is usually because heat cannot leave the heat exchanger fast enough.

1) Verify Temperature Rise

Temperature rise is the difference between supply and return air temperature. Compare it to the nameplate range, commonly 35–65°F for many Carrier models.

1. Run the furnace for 10–15 minutes.
2. Measure return air temp at the return plenum.
3. Measure supply air temp a foot or more downstream of the furnace to avoid radiant heat errors.
4. Delta T = Supply − Return. If above the rated range, airflow is likely low.

If the Delta T is high, proceed to airflow checks. If it is low yet limits still trip, call a professional to assess controls and gas input.

2) Check Air Filter, Coil, and Blower

• Filter: Replace if dirty or if a restrictive filter was recently installed. One‑inch high‑MERV filters can spike static pressure.
• Evaporator coil: A matted A‑coil is a top cause of code 13. Look for dust “felt” on fins. Cleaning typically requires removing the coil cover, protecting the furnace, and using coil cleaner—hire a pro.
• Blower wheel: Dust‑caked blades move less air. Professional removal and cleaning can restore airflow and reduce temperature rise.

3) Confirm Blower Operation and Speed

If the blower starts late, runs slowly, or surges, the furnace can overheat.

• PSC motors: Speed is set by which color lead is connected to HEAT on the control board (black = high speed on many motors; red = low). Moving to a higher speed tap lowers temperature rise. Power off before any change, and label wires.
• ECM/variable‑speed: Adjust via board dip switches or installer setup to ensure proper CFM for heat. Refer to the model’s airflow tables.
• Run capacitor (PSC only): A weak capacitor makes the motor slow and hot. A technician can test and replace (common values 5–15 µF).
• Blower start: Blower delays that are too long can cause short overheating. Verify heat fan‑on delay is set correctly in the installer menu.

4) Evaluate Ductwork and External Static Pressure

High external static pressure (ESP) makes the blower move less air, overheating the furnace.

• Target ESP: Around 0.5 in. w.c. for many PSC systems; up to ~0.8 in. w.c. for some ECM systems, per manufacturer tables.
• Restrictions: Crushed flex duct, long undersized runs, closed dampers, dirty returns, or poorly sized filters increase ESP.
• Test: A technician can measure ESP with a manometer at the return and supply plenums and compare to blower performance charts.

5) Inspect Limit Switches and Wiring

Loose connectors, corroded spade terminals, or damaged wires in the limit circuit can mimic overheating. With power off, reseat connectors and inspect for brittle insulation. If the limit switch is weak or out of calibration, a pro can test continuity versus temperature and replace if needed.

6) Consider Gas Input and Combustion

Overfiring can raise heat exchanger temperatures and trip limits even with good airflow. This step is for licensed technicians.

• Manifold pressure: Typically ~3.5 in. w.c. for natural gas and ~10 in. w.c. for LP, per the nameplate.
• Orifices: Correct size for altitude and gas type.
• Secondary heat exchanger: High resistance from internal blockage on condensing furnaces can cause hot supply air and limit trips; requires professional inspection.

7) Rule Out Control Board Logic Issues

If conditions are normal but code 13 persists, the board could be misinterpreting limit signals. A professional can check error history, sensor readings, and perform component isolation. Board replacement is sometimes required.

Causes And Fixes At A Glance

How To Clear Or Reset Carrier Furnace Code 13

• Automatic reset: Many Carrier controls clear the lockout after roughly three hours once the furnace cools.
• Manual reset: Turn the furnace power switch off for 30 seconds, then back on. Alternatively, set the thermostat to Off for a few minutes, then call for heat again.
• Important: If the underlying issue remains, the limit may trip again and the code will return. Always address airflow or firing causes first.

Preventive Maintenance To Avoid Future Lockouts

• Filters: Replace 1‑inch filters every 1–3 months; 2–4 inch media every 3–6 months. Ensure proper size and fit.
• Professional servicing: Annual heating check to clean burners, verify temperature rise, check ESP, inspect coil and blower, and confirm gas pressure and safeties.
• Ductwork: Keep returns unblocked. If rooms are chronically noisy or weak, have duct design evaluated.
• Coil cleanliness: If you have central AC, schedule coil cleaning every 2–3 years in dusty homes or with shedding pets.
• Thermostat settings: Avoid extreme setbacks that cause long, hot recovery runs, especially in tight duct systems.
• Upgrades: Consider a larger filter cabinet or return duct additions to reduce static pressure and stabilize airflow.

When To Call A Professional And What It May Cost

Call a qualified HVAC technician if the filter and vent checks do not help, the blower behaves erratically, or if code 13 reappears after a reset. Do not attempt gas pressure or advanced electrical diagnostics without proper training.

• Service call and diagnosis: $90–$200 in many U.S. markets.
• Blower cleaning: $150–$350 depending on access and dirt level.
• A‑coil cleaning: $150–$450; more if removal is required.
• Run capacitor (PSC): $100–$250 installed.
• Blower motor replacement: PSC $400–$900 parts; ECM $600–$1,400 parts; installed totals often $700–$1,800.
• Limit switch replacement: $120–$300 installed.
• Duct modifications/add returns: Highly variable; $300–$2,000+ depending on scope.

Related Carrier Codes And What They Mean

• Code 33: Limit or flame rollout open. Often indicates an open limit or rollout switch during operation. Can precede or pair with code 13 if the control goes to lockout.
• Code 24/Code 41/Code 43: Depending on model, blower faults or secondary voltage issues can indirectly cause overheating by limiting airflow.
• Interpretation: Use the furnace’s wiring diagram and fault legend on the blower door for model‑specific meanings; some codes vary by generation.

Carrier, Bryant, And Payne: Same Code, Similar Fixes

Carrier, Bryant, and Payne share many control boards and diagnostics. Error code 13 typically indicates the same limit lockout across these brands. Model‑specific service instructions may differ in blower setup, airflow tables, or fan delays, so consult the exact model literature.

Model‑Specific Tips

• Condensing (90%+ AFUE) furnaces: Secondary heat exchanger and condensate issues can raise temperatures. Ensure the condensate trap is clear and drain lines are not blocked.
• Non‑condensing (80% AFUE) furnaces: Focus on filter, blower, and duct restrictions. Temperature rise range is often wider; confirm on the nameplate.
• Variable‑speed (ECM) models: Check installer setup for heating CFM and temperature rise targets. Some boards allow “comfort” profiles that can reduce airflow; select a profile that meets rise specs.

How Airflow And Temperature Rise Interact

Airflow (CFM) determines how fast heat leaves the heat exchanger. Lower CFM = higher supply air temperature. If supply air exceeds the limit threshold, the switch opens. Most furnaces are sized for about 30–50 BTU per CFM in heating; installers use blower charts to match target rise. When ducts are restrictive, even high blower speed may not deliver needed CFM.

DIY Tools That Help

• Digital thermometer: Measures supply and return temperatures to check rise.
• Static pressure manometer (advanced DIY): With proper training, can measure ESP to confirm duct issues. Many homeowners leave this to pros.
• Flashlight and mirror: To inspect coil and blower areas.
• Camera/phone: Record wiring connections before moving blower speed taps.

Resetting Without Recurring Trips

After replacing a filter or opening registers, reset the furnace by cycling power. Monitor a full heating cycle. A stable temperature rise inside the nameplate range and steady burner operation indicate the root cause was addressed. Repeated trips mean more diagnosis is needed.

Frequently Asked Questions

Is it safe to keep resetting code 13? Occasional resets are fine after correcting airflow. Repeated lockouts without a fix risk heat exchanger damage. Address the cause promptly.

Can a thermostat cause code 13? Indirectly. Aggressive recovery or fan control settings can stress airflow, but thermostats rarely cause overheating by themselves.

Why did code 13 appear after installing a “better” filter? High‑MERV 1‑inch filters can be very restrictive. Switch to a deeper media filter or a lower‑resistance option sized for your system.

Will duct cleaning fix it? Only if ducts are truly blocked with debris. Most limit problems trace to filters, coils, blower wheels, or undersized ducts—not dust alone.

How long until the furnace resets automatically? Many Carrier boards clear a limit lockout after about three hours, but a manual power cycle resets sooner. Fix the cause first.

Key Specs To Check On The Furnace Nameplate

• Temperature rise range: Example 35–65°F (varies by model). Stay within this during operation.
• Input and manifold pressure: Input BTU/hr and natural gas/LP manifold pressure targets.
• Electrical data: Blower motor type and amperage to confirm proper operation.

SEO Notes: Keywords Used Naturally

This guide incorporates relevant phrases such as Carrier furnace code 13, limit switch lockout, high limit switch, Carrier error code 13, temperature rise, and restricted airflow. These reflect how homeowners search for solutions while keeping the language clear and helpful.

Sources And Technical References

• Carrier Residential – Product Literature and Support
• Carrier Furnaces – Model Pages With Manuals
• AHRI Directory – System Matchups And Performance Data
• ASHRAE – Airflow And Duct Design Fundamentals