Carrier 59SC2B Error Code 31: Pressure Switch Did Not Close or Reopened

What Does Code 31 Mean?

Status code 31 (three short flashes, one long flash) on the Carrier 59SC2B means the pressure switch failed to close, or reopened, during a heating attempt. The pressure switch confirms the inducer is pulling enough draft to vent combustion gases safely; when the board never sees it close, it will not allow ignition.

The 59SC2B responds in stages. If a low pressure switch (LPS) stays open longer than five minutes, the inducer shuts off for 15 minutes before retrying. If a 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 3 hours before trying again. That staged behavior is why the fault can look like long pauses or eventual lockout.

The most common cause is a blocked or restricted exhaust vent or intake pipe — snow, ice, leaves, a bird or wasp nest at the termination. Next is restricted condensate drainage: this furnace produces water in normal operation, and a clogged drain or trap can back up and disturb the pressure switch. Both of those are homeowner-checkable. Deeper causes — disconnected or obstructed pressure tubing and a failing inducer motor — are professional repairs. Note that code 31 is the opposite of code 23: code 31 is the switch not closing (or reopening) during a draft problem, whereas code 23 is the switch failing to open when it should.

What You'll Notice

• The inducer/draft motor starts and runs but the furnace never advances to ignition
• The furnace pauses for long stretches (inducer off ~15 minutes) or eventually locks out for 3 hours
• Outside, the exhaust or intake termination is blocked by snow, ice, leaves, or a nest
• The problem is worse on very windy days, or the condensate drain is gurgling or backed up
• Code recall shows a three-short, one-long pattern

Common Causes

How This Is Diagnosed

The check runs from the easy, external causes inward. The vent and intake terminations are inspected and cleared, and the condensate drain and trap are confirmed flowing, since a blockage in either is the usual trigger. If draft still cannot be proven, a technician looks at the pressure tubing for obstruction or trapped condensate, measures inducer voltage and output, and tests the pressure switch itself — the parts of the diagnosis that are not homeowner-safe.

How to Fix It: Clear the Vent, Intake, and Condensate Drain

What You'll Need

• Flashlight
• Gloves
• Wet/dry vacuum (optional, for the condensate drain)

Steps

1. Turn off 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 (perpendicular to the pipe). If you smell gas, leave immediately and call your gas company.
2. Clear the exhaust vent and intake outside Find where the furnace's PVC exhaust and combustion-air intake pipes exit the building and check both terminations for snow, ice, leaves, or nests. Clear any blockage and make sure the pipes are not buried in snow or pressed against a wall or shrub.
3. Clear the condensate drain and trap Locate the small condensate drain line and trap. If water is backing up, flush the line with warm water or use a wet/dry vacuum at the outlet, and clean out any debris in the trap. Standing water here can disturb the pressure switch.
4. Visually inspect the vent piping indoors Look along the vent pipes for obvious sags where water could pool, loose joints, or damage. Just note any problems — do not disassemble the vent piping or the pressure tubing, which are pro repairs.
5. Restore gas and power, then test Turn the gas valve to ON and the breaker to ON. Set the thermostat to call for heat and listen for the inducer to start and the ignition sequence to complete without returning to code 31.

When to Call a Professional

Contact a licensed HVAC technician if:

• The exhaust vent and intake are clear but code 31 keeps returning
• The condensate drain and trap are clear but the code persists
• The inducer motor runs weakly, is noisy, or does not spin up
• The pressure tubing is disconnected, cracked, or full of condensate
• The fault occurs mainly on windy days, suggesting an improper vent termination

✓ Verified against manufacturer service manual — March 2026