Flash Code 33 on Carrier 58STA100-20 Furnace | Causes & Fix

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• Source: Official Carrier service manual
• Reviewed by: Geo Technical Editorial
• Published: March 15, 2026
• Updated: March 15, 2026

Safety notice: Start with user-safe checks only. Do not open sealed refrigerant systems, inspect gas-train components, or work on energized high-voltage parts unless you are qualified to service this appliance.

Error 33 on Carrier : Limit switch open — temperature limit exceeded. Follow the troubleshooting steps below to diagnose and fix this furnace issue. ## What Flash Code 33 Means on a Carrier 58STA100-20 Flash code 33 on the Carrier 58STA100-20 furnace indicates the limit switch or rollout switch has opened due to an over-temperature condition. The limit switch is an automatic-reset thermal safety device located in the heat exchanger supply air plenum. It opens when the discharge air temperature exceeds the limit (typically 175–200 °F / 79–93 °C), cutting the gas valve. The most common cause is restricted airflow — a dirty air filter, a blocked return air duct, or a closed supply register. When 33 appears, the Carrier furnace pauses normal operation to protect internal components from potential damage. Not every 33 fault requires a professional engineer — many are resolved through careful inspection and routine maintenance. Work through each diagnostic step below in sequence, starting with the simplest check, to identify the root cause efficiently before considering part replacement or arranging a service call. ## Symptoms - LED flashing 33 pattern

• Furnace cycles on then off after 1–5 minutes of heating
• Supply air vents very hot during operation ## Common Causes - Limit switch open — temperature limit exceeded ## Tools You May Need - digital multimeter
• replacement filter ## Step-by-Step Repair Guide | Step | Action | Result | |------|--------|--------| | 1 | Check and replace the furnace air filter — a clogged filter is the most common cause of code 33. | Filter replaced — continue to step 2 if the issue persists. | | 2 | Ensure all supply and return air registers are open. | Registers checked — continue to step 3 if the issue persists. | | 3 | Allow furnace to cool for 30 minutes, then attempt restart. | Furnace restarted — continue to step 4 if the issue persists. | | 4 | If code 33 persists after filter replacement, inspect the blower motor — test winding resistance. | Blower motor tested — continue to step 5 if the issue persists. | | 5 | If blower motor is good and code 33 persists, the limit switch may have failed in the open position — replace limit switch. | Limit switch replaced — if the fault remains, contact a qualified technician. | ## Safety Precautions Turn off the furnace before inspecting or replacing the air filter or limit switch. The heat exchanger and limit switch housing can be extremely hot during a code 33 event — allow to cool fully before touching internal components. ## Estimated Repair Time and Difficulty Most furnace flash code 33 faults can be assessed and addressed in 30–90 minutes using the steps above. The diagnostic process requires only basic tools: a multimeter and standard screwdrivers cover the majority of checks described here. If a replacement part is required, allow 1–5 working days for delivery. A trained technician with access to the correct diagnostic equipment will typically resolve this type of fault within a single service visit. ## When to Call a Technician Contact a Carrier authorised service technician if: the fault persists after following all the steps above; if PCB-level or refrigerant-circuit work is required; or if you are not comfortable working with electrical or mechanical components. When calling, have the full model number, the error code displayed, and a brief description of any sounds or symptoms you noticed before the fault appeared — this helps the technician prepare the correct parts and tools. ## Frequently Asked Questions Q: How often should I replace the furnace air filter? Every 1–3 months with standard 1-inch filters. Every 6–12 months with 4-inch media filters. Check monthly during peak heating/cooling season. Q: Can code 33 damage the heat exchanger? Repeated over-temperature cycling can stress the heat exchanger and shorten its life. Address code 33 promptly by fixing the airflow restriction. Q: Is the limit switch the same as the rollout switch? No — the limit switch protects against over-temperature in the supply air stream; the rollout switch protects against flame rollout. Both can trigger code 33 on Carrier 58STA models. ### Is it safe to keep using my Carrier with error 33? If error 33 does not indicate an immediate safety hazard such as a gas leak, electrical short, or water flooding, you can continue limited use while monitoring the appliance. However, running the furnace with an active error code increases the risk of further damage. Address the underlying issue as soon as possible. For high-severity errors, disconnect the appliance and contact a qualified technician. ### How much does it cost to fix error 33 on a Carrier ? Repair costs for error 33 on the Carrier furnace depend on the root cause. DIY fixes such as cleaning filters, resetting the unit, or reconnecting loose wires cost nothing beyond basic tools. If a component replacement is needed, parts and labor from a certified technician are the main expenses. Contact a Carrier-authorized service provider for an accurate estimate based on your specific situation. ### What does error code 33 mean on a Carrier furnace? Error code 33 on Carrier furnace models indicates Limit switch open — temperature limit exceeded. This error is detected by the appliance’s internal diagnostic system and displayed to alert the user that attention is needed. Refer to the troubleshooting steps above for guidance on resolving this issue. ### Is it safe to keep using my Carrier with error 33? If error 33 does not indicate an immediate safety hazard such as a gas leak, electrical short, or water flooding, you can continue limited use while monitoring the appliance. However, running the furnace with an active error code increases the risk of further damage. Address the underlying issue as soon as possible. For high-severity errors, disconnect the appliance and contact a qualified technician. ### How much does it cost to fix error 33 on a Carrier ? Repair costs for error 33 on the Carrier furnace depend on the root cause. DIY fixes such as cleaning filters, resetting the unit, or reconnecting loose wires cost nothing beyond basic tools. If a component replacement is needed, parts and labor from a certified technician are the main expenses. Contact a Carrier-authorized service provider for an accurate estimate based on your specific situation. ### What does error code 33 mean on a Carrier furnace? Error code 33 on Carrier furnace models indicates Limit switch open — temperature limit exceeded. This error is detected by the appliance’s internal diagnostic system and displayed to alert the user that attention is needed. Refer to the troubleshooting steps above for guidance on resolving this issue.