Flash Code 43 on Carrier 58STA080-16 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 43 on Carrier : Condensate drain blocked (90%+ AFUE models). Follow the troubleshooting steps below to diagnose and fix this furnace issue. ## What Flash Code 43 Means on a Carrier 58STA080-16 Flash code 43 on the Carrier 58STA080-16 high-efficiency furnace indicates the condensate drain is blocked. 90%+ AFUE condensing furnaces extract latent heat from combustion gases, which causes the gases to condense into water (condensate). This water must drain continuously via the condensate drain line. If the drain line is blocked (by algae, debris, or ice), condensate backs up into the heat exchanger, which causes the pressure switch to activate and prevents further operation. When 43 appears, the Carrier furnace pauses normal operation to protect internal components from potential damage. Not every 43 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 43 pattern

• Furnace not starting
• Water pooling near the furnace base ## Common Causes - Condensate drain blocked (90%+ AFUE models) ## Tools You May Need - wet/dry vacuum
• flexible brush or pipe cleaner
• white vinegar ## Step-by-Step Repair Guide | Step | Action | Result | |------|--------|--------| | 1 | Turn off the furnace. Locate the condensate drain trap and drain hose. | Drain located — continue to step 2 if the issue persists. | | 2 | Use a wet/dry vacuum to clear the drain trap of standing water and debris. | Trap cleared — continue to step 3 if the issue persists. | | 3 | Flush the drain line with warm water or dilute white vinegar to clear algae. | Line flushed — continue to step 4 if the issue persists. | | 4 | Ensure the drain hose has a downward slope and is not kinked. | Drain slope confirmed — continue to step 5 if the issue persists. | | 5 | Restore power and test the furnace. | Code 43 cleared — if the fault remains, contact a qualified technician. | ## Safety Precautions Turn off the furnace before cleaning the condensate drain. Do not use harsh chemical drain cleaners — they can damage PVC drain components and the condensate trap. ## Estimated Repair Time and Difficulty Most furnace flash code 43 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 clean the condensate drain? Once per heating season. Monthly treatment with dilute white vinegar prevents algae growth that causes code 43. Q: Can code 43 cause water damage? Yes — a blocked condensate drain can overflow the trap and flood the furnace base or the surrounding floor. Address code 43 promptly. Q: Does code 43 only affect condensing furnaces? Yes — code 43 (condensate drain) only applies to 90%+ AFUE condensing models. Standard 80% AFUE models like the 58STA080 do not produce condensate. ### Is it safe to keep using my Carrier with error 43? If error 43 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 43 on a Carrier ? Repair costs for error 43 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 43 mean on a Carrier furnace? Error code 43 on Carrier furnace models indicates Condensate drain blocked (90%+ AFUE models). 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 43? If error 43 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 43 on a Carrier ? Repair costs for error 43 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 43 mean on a Carrier furnace? Error code 43 on Carrier furnace models indicates Condensate drain blocked (90%+ AFUE models). 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.