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

What Does Code 31 Mean?

Code 31 on the Carrier 59SC5A means the pressure switch did not close when the inducer motor started, or it closed and then reopened during the heating cycle. The pressure switch is a safety device that verifies the inducer motor is producing enough airflow to safely vent combustion gases out of your home. Without this confirmation, the furnace will not allow gas to flow.

If the low pressure switch (LPS) remains open for one minute after the gas valve closes (after three successive attempts), the furnace control locks out for 3 hours before retrying. This prevents the furnace from repeatedly attempting to operate with an unsafe venting condition.

The most common causes are physical obstructions in the venting system. A blocked exhaust vent (from ice, bird nests, leaves, or debris) is the most frequent culprit. A clogged condensate drain can also cause this — when condensate water backs up, it can block the pressure port on the inducer housing. Disconnected, kinked, or water-logged pressure tubing between the inducer and the pressure switch is another common cause. Excessive wind can also temporarily trigger this code by creating enough back-pressure to keep the switch from closing.

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 find where the furnace exhaust vent exits your home (typically a PVC pipe through a side wall or roof). Check for 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 59SC5A uses a two-pipe direct vent system (separate intake and exhaust pipes), also check 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 it is clogged, water can back up into the inducer housing and block the pressure switch port. Verify water flows freely through the drain. If it appears clogged, 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 find the small rubber or silicone tube(s) running from the inducer housing to the pressure switch(es). Check that the tubing is connected at both ends, not kinked, cracked, or clogged. If there is water in the tubing, gently disconnect it, drain the water, and 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 fires 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, is grinding, or does not start at all
• The code keeps recurring even after clearing vent obstructions
• You suspect the pressure switch itself is defective
• The issue consistently occurs during windy weather (a wind guard may need to be installed)