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

What Does Code 31 Mean?

Status code 31 on the Carrier 59SC5A means the pressure switch did not close, or it reopened, during the draft-proving stage. The pressure switch is a safety device that confirms the inducer motor is pulling enough draft to vent combustion gases safely out of your home before the burners are allowed to fire.

The board responds according to which switch is involved. If the low pressure switch (LPS) stays open 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, and this repeats over three successive trials, the control locks out for three hours before retrying.

The most common cause is a blocked or restricted exhaust vent or combustion-air intake, snow, ice, leaves, or a nest at the outdoor termination. The next most common is restricted or blocked condensate drainage, since a condensing furnace makes water and a clogged drain can back up and disturb the pressure system. Disconnected or obstructed pressure tubing and a defective inducer motor are also possible but are professional repairs. This is the opposite situation from code 23, where the pressure switch fails to open after draft stops; here the switch fails to close (or holds closed then reopens) during draft proving.

What You'll Notice

• The inducer motor runs but the burners never light, and code 31 is displayed
• The furnace short-cycles or pauses for long periods (about 15 minutes) between attempts before retrying
• The exhaust or intake termination outside is blocked by snow, ice, leaves, or a nest
• Water is backing up around the furnace or the condensate drain looks clogged
• The code shows up mainly on very windy days

Common Causes

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

What You'll Need

• Flashlight
• Wet/dry vacuum
• Gloves

Steps

1. Turn off electrical power at the breaker and shut off the gas supply valve Flip the furnace circuit breaker to OFF and turn the manual gas shutoff valve to the OFF position (handle perpendicular to the pipe). If you smell gas, leave immediately and call your gas company.
2. Inspect the exhaust and intake terminations outside Find where the PVC exhaust and combustion-air intake pipes exit your home. Clear any snow, ice, leaves, or nests, and make sure the terminations are not buried in snow or pressed against a wall or shrub. Blocked terminations are the leading cause of code 31.
3. Check the condensate drain Locate the small condensate drain line and confirm water is not backing up. If accessible, clear it by pouring warm water through it or using a wet/dry vacuum at the drain outlet, and check the condensate trap for debris if the furnace has one.
4. Look over the visible vent piping inside Follow the vent pipes from the furnace to the exterior and note any sagging sections holding water, obvious disconnected joints, or damage. Do not disassemble the piping or pressure tubing; just record what you see for a technician if there is a problem.
5. Restore power and gas, then test Turn the gas valve back to ON and the breaker to ON. Set the thermostat to call for heat and listen for the inducer to run steadily, then watch that the burners light and the cycle completes.

When to Call a Professional

Contact a licensed HVAC technician if:

• The exhaust and intake are clear but code 31 persists
• You cannot safely access or clear the condensate drain
• The inducer motor is unusually loud, hums, or does not start
• Water is pooling around the base of the furnace
• The code appears mainly on windy days, which can indicate an improper vent termination or sizing

✓ Verified against manufacturer service manual — March 2026