Carrier 59SC5A Error Code 31: Pressure Switch Did Not Close or Reopened

What Does Code 31 Mean?

Status code 31 on the Carrier 59SC5A indicates that the pressure switch did not close or reopened during operation. The pressure switch is a safety device that verifies the inducer motor is creating enough draft to safely vent combustion gases out of your home.

If the low pressure switch (LPS) opens for longer than five minutes, the inducer shuts off for 15 minutes before retrying. If the high pressure switch (HPS) remains open for one minute after the gas valve closes (after three successive trials), the furnace control will lock out for 3 hours before retrying.

The most common cause is a blocked or restricted exhaust vent or intake pipe. Snow, ice, debris, bird nests, or even a wasp nest can block the vent pipes where they exit your home. The second most common cause is restricted or blocked condensate drainage — condensing furnaces produce water during operation, and if the drain is clogged, it can back up and interfere with the pressure switch. Other possible causes include disconnected or obstructed pressure tubing and a defective inducer motor.

Common Causes

How to Fix It: Clear Vent Blockages and Check Condensate Drain

What You'll Need

• Flashlight
• Wet/dry vacuum or small cup for condensate
• Gloves

Steps

1. Turn off electrical power at the breaker and shut off the gas supply valve Locate the furnace circuit breaker and flip it to OFF. Turn the gas shutoff valve to the OFF position (perpendicular to the pipe). If you smell gas, leave immediately and call your gas company.
2. Inspect the exhaust vent and intake pipe outside Go outside and locate where the furnace exhaust vent and combustion air intake pipe exit your home (usually PVC pipes on an exterior wall). Check for any blockages such as snow, ice, leaves, bird nests, insect nests, or other debris. Clear any obstructions you find. Make sure the vent terminations are not buried in snow or pressed against a wall or shrub.
3. Check the condensate drain Locate the condensate drain line — a small PVC pipe that carries water away from the furnace. Check if water is backing up or if the drain is clogged. If accessible, try clearing the drain by pouring warm water through it or using a wet/dry vacuum on the drain outlet. Also check the condensate trap if your furnace has one — it may be full of debris.
4. Inspect the visible vent piping inside Look along the vent pipes from the furnace to where they exit the building. Check for any obvious sagging sections where water could collect, disconnected joints, or damage. Do not disassemble the vent piping — just note any problems.
5. Restore power and gas, then test Turn the gas supply valve to ON and flip the circuit breaker to ON. Set your thermostat to call for heat and monitor the furnace. Listen for the inducer motor to start and watch for the ignition sequence to complete successfully.

When to Call a Professional

Contact a licensed HVAC technician if:

• The exhaust vent and intake appear clear but code 31 persists
• You cannot access or clear the condensate drain
• The inducer motor sounds unusually loud or does not start
• You see water pooling around the furnace base
• The code occurs only on windy days (may indicate improper vent termination)

✓ Verified against manufacturer service manual — March 2026