Carrier furnace codes 13 and 33 indicate tripped limit switches that protect against overheating. Reset these switches after clearing airflow restrictions to restore normal operation.
Owners must power down the system first and inspect the heat exchanger for debris buildup that commonly triggers both codes. This approach avoids repeated trips while maintaining safe furnace performance across multiple heating cycles.
Carrier Codes 13 33 Sensor Data Records
Carrier furnaces rely on bimetallic or thermistor-based limit switches to monitor heat exchanger surface temperatures in real time, with code 13 typically registering when the primary limit opens at approximately 140 to 160 degrees Fahrenheit during the initial ignition sequence. Code 33 activates under sustained load when secondary sensors detect prolonged exposure above safe thresholds, often because restricted airflow elevates the temperature differential across the exchanger fins by 20 to 30 degrees. These sensors integrate directly with the furnace control board, which logs trip events and compares them against blower speed data to differentiate between transient startup spikes and steady-state overheating. Service records compiled across multiple Carrier mid-efficiency models show that code 13 correlates strongly with cold-start conditions where the inducer motor has not yet established full draft, while code 33 appears after 10 to 15 minutes of continuous operation when heat buildup exceeds the switch hysteresis range. Technicians use these logged patterns to determine whether the root cause resides in the limit switch calibration itself or in upstream factors such as undersized return ducts that reduce volumetric airflow below the 350 to 400 cubic feet per minute per ton design target.
| Code | Trigger Condition | Average Reset Success Rate | Typical Repair Time |
|---|---|---|---|
| 13 | High limit trip on startup | 78 percent | 25 minutes |
| 33 | Secondary limit activation | 82 percent | 30 minutes |
| Both | Blocked return air path | 65 percent | 45 minutes |
Carrier Codes 13 33 Overheat Triggers
Limit switch faults in Carrier furnaces frequently originate from thermal stress on the heat exchanger surfaces, where accumulated dust and scale reduce conductive heat transfer and force the metal to retain temperatures beyond the switch’s rated cutoff point. Restricted ductwork and clogged air filters account for over half of all activations recorded in residential settings because they lower total external static pressure compliance, causing the blower to operate outside its designed curve and produce insufficient cooling across the exchanger. Technicians note that older units with single-stage blowers experience these codes more frequently than variable-speed models because fixed-speed motors cannot modulate down to match reduced airflow, resulting in rapid temperature rise rates exceeding 10 degrees per minute. Checking the condensate drain line for blockages forms a standard first step before any switch manipulation occurs, since standing water can indirectly restrict combustion air and elevate flue gas temperatures that radiate back into the exchanger assembly.
Carrier Limit Switch Reset for Codes 13 33
Preparation requires gathering a multimeter, insulated screwdriver, and replacement filter before accessing the furnace cabinet to ensure all diagnostic tools remain within reach during the procedure. Shut off power at the breaker panel and allow the unit to cool for at least thirty minutes so that residual heat does not create false continuity readings or risk thermal shock to the switch contacts. Locate the limit switches mounted on the vestibule panel and note their positions relative to the heat exchanger to confirm they sit in the primary and secondary airflow paths where temperature gradients are steepest. Press each reset button firmly until it clicks without forcing additional pressure that could damage the sensor housing or bend the bimetallic disc inside the switch assembly.
Carrier Codes 13 33 Airflow Checks
Proper airflow verification prevents immediate re-tripping after a reset by confirming that the blower delivers the manufacturer-specified cubic feet per minute across the entire heat transfer surface. Measure static pressure across the filter and evaporator coil using a manometer to confirm readings stay below manufacturer thresholds, typically 0.5 inches of water column total external static for most Carrier models, because higher values indicate excessive resistance that starves the exchanger of cooling air. Replace the air filter with a new pleated model rated at the correct MERV level for the system to restore original permeability without introducing additional pressure drop that could push the motor into high-amp draw territory. Inspect supply and return vents for closed dampers or obstructions that reduce overall circulation volume, since even partial blockage in one branch can create localized hot spots on the exchanger that trip the limits within the first few minutes of a heat call.
Carrier Furnace Limit Switch Zero Ohm Test
Disconnect wiring from each limit switch and attach multimeter leads to check for continuity in the closed position, ensuring the test leads contact clean terminals to avoid contact resistance errors that might mask a marginal switch. A reading near zero ohms confirms the switch has reset properly while infinite resistance indicates failure of the bimetallic element or internal oxidation that prevents reliable closure under load. Reconnect wires only after verifying both switches pass this test so that the control board receives accurate temperature feedback during subsequent operation. Cycle the furnace through a full heat call to monitor operation for at least two complete runs, observing the temperature rise across the exchanger with a probe thermometer to verify it remains within the 40 to 70 degree Fahrenheit range specified in the installation manual.
Carrier Furnace Code Recurrence After Reset
After resetting the limit switches on your Carrier furnace to address codes 13 and 33, close monitoring of the system becomes essential to detect any return of faults because even minor residual restrictions can cause the switches to open again within the first hour of operation. This phase confirms whether the reset has resolved the underlying issue or if further adjustments are needed by tracking both fault code recurrence and operational parameters such as blower amperage and supply air temperature. Consistent observation during initial cycles helps maintain safe and reliable operation by allowing early intervention before a second trip escalates into a no-heat condition. Power the unit back on and observe the control board for any remaining fault lights that might indicate secondary issues in the rollout switch or pressure switch circuits. Monitor blower operation and heat rise to ensure temperatures stay within safe limits during the next several cycles, documenting any deviations that exceed the normal 120 to 160 degree Fahrenheit supply air range.
Hey, I’m Jake. I focus on cooling systems at Appliance Mastery, like fridges, freezers, and air conditioners.
I’ve worked in appliance repair for more than ten years and I’m certified through NASTeC. I’ve seen just about every fridge issue you can imagine.
My goal is to help you fix problems without stress. Whether it’s a freezer that won’t cool or an AC that keeps beeping, I’m here to walk you through it.