Flash Code 31 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 31 on Carrier : Pressure switch fault — inducer or flue problem. Follow the troubleshooting steps below to diagnose and fix this furnace issue. ## What Flash Code 31 Means on a Carrier 58STA100-20 Flash code 31 on the Carrier 58STA100-20 furnace indicates the pressure switch has not closed, preventing the furnace from advancing to ignition. The pressure switch monitors that the inducer motor is creating the correct negative pressure in the vent system before gas is admitted. If the inducer motor is faulty, the flue is blocked, or the condensate drain is clogged (on 90%+ AFUE models), the pressure switch will not close and code 31 is displayed. When 31 appears, the Carrier furnace pauses normal operation to protect internal components from potential damage. Not every 31 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 31 pattern

• Furnace does not advance to ignition
• Inducer motor running but pressure switch fault ## Common Causes - Pressure switch fault — inducer or flue problem ## Tools You May Need - digital multimeter
• rubber tubing (to blow-test pressure switch) ## Step-by-Step Repair Guide | Step | Action | Result | |------|--------|--------| | 1 | Turn off furnace. Listen — does the inducer motor run when furnace attempts to start? | Inducer motor operation confirmed — continue to step 2 if the issue persists. | | 2 | If inducer runs: inspect pressure switch hose (rubber tube from inducer to switch) for cracks or disconnection. | Hose inspected — continue to step 3 if the issue persists. | | 3 | Gently blow into the pressure switch hose — it should click audibly as pressure is applied. | Switch click confirmed or absent — continue to step 4 if the issue persists. | | 4 | If switch does not click when blown: replace pressure switch. | Switch replaced — continue to step 5 if the issue persists. | | 5 | If inducer does not run: test inducer motor winding resistance and replace if open-circuit. | Inducer tested — if the fault remains, contact a qualified technician. | ## Safety Precautions Never drill or remove pressure switch hoses while the furnace is operating. The vent system carries combustion gases. Turn off the furnace and gas before inspecting the flue or pressure switch hoses. ## Estimated Repair Time and Difficulty Most furnace flash code 31 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: Can a blocked flue cause code 31? Yes — a partially or fully blocked vent pipe prevents the inducer from creating adequate draft, so the pressure switch does not close. Q: Can condensate blockage cause code 31? Yes — on 90%+ AFUE (condensing) furnaces, a blocked condensate drain can flood the heat exchanger and prevent the pressure switch from sensing correct draft. Q: Is code 31 dangerous? Code 31 prevents ignition, which is safe. However, if the flue is blocked, do not attempt to operate the furnace until the blockage is cleared. ### Is it safe to keep using my Carrier with error 31? If error 31 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 31 on a Carrier ? Repair costs for error 31 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 31 mean on a Carrier furnace? Error code 31 on Carrier furnace models indicates Pressure switch fault — inducer or flue problem. 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 31? If error 31 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 31 on a Carrier ? Repair costs for error 31 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 31 mean on a Carrier furnace? Error code 31 on Carrier furnace models indicates Pressure switch fault — inducer or flue problem. 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.