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

What Does Code Code 31 Mean?

Code 31 on the Carrier 59SC2B means the pressure switch did not close when expected, or it closed and then reopened. The pressure switch is a safety device that confirms the inducer motor is creating enough airflow to safely vent combustion gases out of your home. If the switch doesn't close, the furnace will not ignite.

The Carrier 59SC2B uses both a low pressure switch (LPS) and may use a high pressure switch (HPS). If the LPS stays open for more than five minutes, the inducer shuts off for 15 minutes before retrying. If the HPS remains open for one minute after the gas valve closes (after three successive attempts), the furnace locks out for 3 hours.

The most common causes relate to airflow problems in the venting system: a blocked or restricted exhaust vent (ice, bird nests, debris), a clogged condensate drain that backs water up into the inducer housing, or disconnected/kinked pressure tubing between the inducer and the pressure switch. Excessive wind conditions can also temporarily cause this code by creating backdraft pressure against the vent termination.

Common Causes

How to Fix It: Check Vent Pipe, Condensate Drain, and Pressure Tubing

What You'll Need

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

Steps

1. Turn off power at the breaker and shut off the gas supply valve Locate the circuit breaker for your furnace and flip it to OFF. Find the gas shut-off valve on the gas line leading to the furnace and turn it to the closed position. If you smell gas at any point, leave the house immediately and call your gas company from outside.
2. Inspect the exhaust vent termination outside Go outside and locate where the furnace exhaust vent exits your home (typically a PVC pipe through a side wall or roof). Check for any obstructions: ice buildup, bird nests, leaves, insect nests, or snow drifts. Clear any debris you find. Make sure the vent screen (if present) is not clogged.
3. Check the intake air pipe (if applicable) If your 59SC2B has a two-pipe venting system (separate intake and exhaust), also inspect the intake air pipe termination outside for the same types of obstructions.
4. Check the condensate drain Locate the condensate drain line and trap at the bottom of the furnace. If the drain is clogged, water can back up into the inducer housing and block the pressure switch port. Check that water flows freely through the drain. If it appears clogged, you can try clearing it with a wet/dry vacuum applied to the drain outlet.
5. Visually inspect the pressure switch tubing Open the furnace cabinet and locate the small rubber or silicone tube(s) running from the inducer housing to the pressure switch(es). Check that the tubing is fully connected at both ends, not kinked, cracked, or clogged with moisture. If the tubing has water in it, gently remove it and drain the water, then reattach it securely.
6. Restore power and test Turn the gas supply back on, then turn on the breaker. Set the thermostat to call for heat. Listen for the inducer motor to start, followed by ignition within 30-60 seconds. If the furnace ignites and runs a full cycle, the issue is resolved.

When to Call a Professional

Contact a licensed HVAC technician if:

• The vent pipe and condensate drain are clear but the code persists
• The inducer motor sounds unusually loud, grinding, or does not run at all
• The code appears repeatedly even after clearing vent obstructions
• You suspect the pressure switch itself is defective
• The issue occurs consistently during windy conditions (may need a vent wind guard installed)