Carrier Error Code 31: Pressure Switch Fault
Complete troubleshooting guide for Carrier furnace error code 31. Learn what causes pressure switch faults, how to diagnose the problem, and when to call a professional.
Pressure switch failed to close or opened unexpectedly, indicating inadequate airflow or draft through the furnace.
Blocked vent pipes (40-50%) or clogged condensate drain lines (20-30%) preventing proper pressure switch operation.
Simple fixes: 30-60 minutes. Complex repairs (inducer motor, heat exchanger): 2-4 hours.
Understanding Carrier Error Code 31
Carrier error code 31 indicates a Pressure Switch Fault, meaning the low or high pressure switch failed to close when expected, or opened unexpectedly during the heating cycle. The pressure switch is a critical safety component that monitors airflow and draft through your furnace's heat exchanger and venting system.
Before your Carrier furnace ignites, the inducer motor must create negative pressure (draft) that pulls combustion air through the burners and pushes exhaust gases safely outside. The pressure switch detects this draft and closes its contacts to signal the control board that proper airflow exists. Only then will the gas valve open and ignition occur.
Code 31 appears when the pressure switch fails to close within the expected timeframe (typically 30-90 seconds), or if it opens unexpectedly during operation. This safety lockout prevents the furnace from running without adequate combustion air or proper venting—conditions that could cause incomplete combustion, carbon monoxide production, overheating, or flame rollout.
⚠️ Safety Warning
Code 31 is a safety lockout. Never bypass the pressure switch or repeatedly reset the system without diagnosing and fixing the underlying cause. Blocked vents, especially, can cause dangerous carbon monoxide buildup.
10 Common Causes of Code 31
1. Blocked Vent Pipes (40-50% of Cases)
Snow, ice, debris, bird nests, or improper termination cap installation can block PVC vent pipes. Even partial blockages restrict exhaust flow, preventing adequate draft. Winter weather is the most common trigger—heavy snow can completely cover exterior vents.
2. Clogged Condensate Drain or Trap (20-30%)
High-efficiency Carrier furnaces produce acidic condensate that drains through a trap. Algae, mold, debris, or frozen water can clog the trap or drain line, causing water backup that blocks the pressure switch sensing port or disrupts airflow. This is especially common if the trap hasn't been cleaned in 2+ years.
3. Dirty Air Filter (15-20%)
A severely clogged air filter restricts return airflow, forcing the system to work harder and reducing the inducer motor's ability to create proper draft. Filters should be checked monthly and replaced every 1-3 months depending on home conditions.
4. Plugged Inducer Motor Nipple (10-15%)
The small metal port where the pressure switch hose connects to the inducer housing can become completely clogged with carbon deposits and debris. This prevents the pressure switch from sensing draft even when the inducer motor runs perfectly. It's one of the most overlooked causes because the opening is tiny (about 1/8 inch).
5. Weak or Failing Inducer Motor (5-10%)
The inducer motor creates the draft required for pressure switch closure. A motor with worn bearings, damaged wheel, or weak capacitor can't generate sufficient negative pressure. Signs include loud grinding noise, slow startup, excessive vibration, or visible damage.
6. Cracked or Kinked Pressure Switch Hose (5-10%)
The rubber or silicone tubing connecting the inducer motor to the pressure switch can crack with age, develop holes, or become kinked. This prevents accurate pressure sensing. Inspect the entire length of tubing for damage, especially near connection points where it may have been pinched during previous service.
7. Defective Pressure Switch (5-10%)
The pressure switch itself can fail due to electrical fatigue, water damage, or manufacturing defects. Switches are designed to close at a specific pressure differential (typically 0.5-1.5 inches of water column). Over time, the diaphragm can weaken or contacts can corrode, preventing proper operation even with adequate draft.
8. Improper Vent Pipe Installation (3-5%)
Vent pipes with excessive length, too many elbows, inadequate slope for condensate drainage, or wrong pipe diameter can prevent proper draft. The furnace installation manual specifies maximum vent lengths and elbow counts. Non-compliant installations may work initially but fail over time as carbon deposits accumulate or condensate freezes.
9. Cracked Heat Exchanger (1-3%)
A cracked heat exchanger disrupts the pressure differential required for proper draft, causing code 31. This is a serious condition requiring immediate furnace shutdown and professional inspection. Signs include visible cracks during inspection, soot buildup, strong odors, or previous overheating issues. Heat exchangers typically last 15-20 years before cracking.
10. Loose Wire Connections (1-3%)
Loose, corroded, or damaged wire connections at the pressure switch terminals can cause intermittent code 31. Vibration from the inducer motor can loosen terminals over time. Inspect connections for tightness, corrosion, or burnt wire insulation.
8 DIY Troubleshooting Steps
⚡ Safety First
Before troubleshooting, turn off power to the furnace at the breaker or disconnect switch. Wait 60 seconds for the control board to fully power down. Never work on a furnace with power connected.
- Step 1: Replace the Air Filter
Check your air filter. If it's visibly dirty, clogged with dust, or hasn't been changed in 2+ months, replace it immediately. A dirty filter is the easiest and cheapest fix for code 31. After replacement, reset the furnace and test. Use high-quality pleated filters (MERV 8-11) for best performance. - Step 2: Inspect Exterior Vent Pipes
Go outside and locate your furnace's PVC vent termination (usually 2-3 white pipes on an exterior wall or roof). Clear away snow, ice, leaves, or debris within 12 inches of the vent opening. Check that the vent cap is intact and not blocked by bird nests or ice. Verify both intake and exhaust pipes are clear. - Step 3: Check for Standing Water
Look around the base of your furnace for standing water or moisture, which indicates condensate drain backup. If present, the drain line or trap is likely clogged. You can try pouring a cup of warm water mixed with bleach into the condensate drain opening to clear minor clogs. If water doesn't drain, professional cleaning is needed. - Step 4: Listen to the Inducer Motor
Call for heat at your thermostat and listen to the furnace startup sequence. The inducer motor should start within 5-10 seconds and run smoothly for 30-60 seconds before you hear the igniter click. It should sound like a small fan motor—quiet and steady. Loud grinding, squealing, or no sound at all indicates inducer motor problems requiring professional service. - Step 5: Inspect Pressure Switch Tubing
With power off, locate the pressure switch (small round device with rubber hose). Follow the hose to the inducer motor housing. Check the entire length for cracks, holes, kinks, or loose connections. The hose should be firmly attached at both ends. If damaged, replacement tubing is inexpensive at HVAC supply stores. - Step 6: Check the Inducer Motor Nipple
If accessible, disconnect the pressure switch hose from the metal nipple on the inducer motor housing (power must be off). Look into the nipple opening with a flashlight. If clogged with debris, carefully insert a small wire or pipe cleaner to clear it. Blow compressed air through the opening to remove remaining debris. Reconnect the hose securely. - Step 7: Verify Wire Connections
With power off, inspect the two wires connected to the pressure switch terminals. Gently tug each wire to ensure it's firmly connected. Look for corrosion, burnt insulation, or loose terminals. If terminals are corroded, they should be cleaned or replaced by a professional. Do NOT attempt to bypass the switch with jumper wires. - Step 8: Reset and Test
After completing steps 1-7, restore power to the furnace. Wait 60 seconds, then set your thermostat 5 degrees above current temperature to call for heat. Listen to the startup sequence. If code 31 returns immediately, the problem requires professional diagnosis with specialized tools (manometers, multimeters, etc.).
Prevention Tips
- ✓Monthly Filter Checks: Inspect your air filter monthly during heating season. Replace when visibly dirty or every 1-3 months depending on home conditions (pets, allergies, dusty environment).
- ✓Annual Condensate Trap Cleaning: Have your HVAC technician clean the condensate trap and drain line during annual maintenance. This prevents 20-30% of code 31 occurrences.
- ✓Winter Vent Inspections: Check exterior vents after heavy snowfall. Clear snow buildup immediately to prevent blockages. Ensure proper clearance and install vent pipe extensions if needed.
- ✓Professional Maintenance: Schedule annual furnace inspections before heating season. Technicians test pressure switches, clean inducer motors, verify vent integrity, and catch problems before they cause lockouts.
- ✓Vent Pipe Protection: Install vent pipe guards or screens to prevent bird nests and debris. Ensure proper slope for condensate drainage. Verify installation meets manufacturer specifications.
- ✓Monitor Furnace Sounds: Learn your furnace's normal startup sequence. Unusual sounds (grinding, squealing, clicking) often precede failures. Address strange noises immediately.
When to Call a Professional
Call a professional HVAC technician immediately if you experience any of these conditions:
- 🚨Gas Odor: If you smell gas (rotten egg odor), evacuate immediately and call 911 or your gas company. Do NOT attempt troubleshooting.
- 🚨Carbon Monoxide Symptoms: Headaches, dizziness, nausea, or flu-like symptoms when furnace runs. Evacuate and call emergency services.
- 🚨Repeated Code 31: If code 31 returns immediately after DIY troubleshooting, professional diagnosis is required.
- 🚨Inducer Motor Failure: Loud grinding, no operation, or excessive vibration indicates motor replacement needed.
- 🚨Cracked Heat Exchanger Suspected: Soot buildup, strong odors, or visible cracks require immediate professional inspection.
- 🚨Complex Drain Issues: If you can't access or clear the condensate trap, professional service is needed.
- 🚨Electrical Testing Required: Testing pressure switch operation, control board signals, or inducer motor capacitor requires specialized tools and training.
Professional HVAC technicians have specialized tools for code 31 diagnosis: digital manometers to measure exact pressure switch closing points, multimeters to test switch and motor continuity, inducer motor amp draw meters to detect bearing wear, and inspection cameras to check vent pipe integrity. These tools ensure accurate diagnosis rather than guessing and replacing parts.
Frequently Asked Questions
What is Carrier error code 31?
Carrier error code 31 indicates a Pressure Switch Fault, meaning the low or high pressure switch failed to close when expected or opened unexpectedly during operation. The pressure switch is a critical safety device that monitors proper airflow and draft through the furnace. When it detects insufficient or excessive pressure, it prevents ignition to avoid safety hazards like incomplete combustion or carbon monoxide production.
What does the pressure switch do in a Carrier furnace?
The pressure switch monitors airflow through the heat exchanger and venting system by detecting negative pressure created by the inducer motor. It must close (complete the circuit) before the gas valve opens, proving adequate combustion air supply and proper exhaust draft. This safety mechanism ensures dangerous combustion gases are safely vented outside rather than accumulating in your home or causing incomplete combustion.
What causes Carrier error code 31?
The most common causes are blocked vent pipes (40-50% of cases), clogged condensate drain lines or traps (20-30%), dirty air filters restricting airflow (15-20%), weak inducer motor unable to create sufficient draft, plugged metal nipple where the pressure switch hose connects to the inducer, cracked or kinked pressure switch tubing, defective pressure switch requiring replacement, or in rare cases, a cracked heat exchanger preventing proper draft.
Can I fix Carrier error code 31 myself?
You can troubleshoot basic issues: replace dirty air filters, visually inspect vent pipes for obvious blockages (snow, bird nests, debris), clean accessible condensate drain lines, and inspect pressure switch tubing for damage. However, testing the pressure switch with a multimeter, cleaning the condensate trap assembly, diagnosing inducer motor issues, or checking for cracked heat exchangers requires professional HVAC tools, training, and safety equipment. Attempting complex repairs can void warranties or create safety hazards.
How do I reset Carrier error code 31?
Code 31 typically resets automatically once the underlying issue is resolved. To manually reset: turn off power to the furnace at the breaker or disconnect switch, wait 60 seconds for the control board to fully power down, restore power, and set your thermostat to call for heat. If code 31 returns immediately, the root cause hasn't been fixed—repeated resets without repair can damage components and create safety risks.
Is Carrier error code 31 dangerous?
Code 31 itself is a safety lockout preventing dangerous operation. It stops the furnace from running without proper airflow, which could cause incomplete combustion, carbon monoxide production, overheating, or flame rollout. The underlying causes—especially cracked heat exchangers or blocked vents—can be dangerous if ignored. Never bypass the pressure switch or repeatedly reset without fixing the problem. If you smell gas or suspect carbon monoxide, evacuate immediately and call 911.
Why does Carrier error code 31 appear in winter?
Cold weather creates perfect conditions for code 31: vent pipes can become blocked by snow, ice, or frost buildup; increased furnace runtime leads to more condensate production that can freeze in drain lines; extreme cold stresses the inducer motor making marginal failures more apparent; wind can affect draft pressure especially with improperly installed vents; and frozen condensate traps prevent proper drainage, causing pressure switch issues. These problems rarely occur in mild weather but become critical during heating season.
How do I check if my vent pipe is blocked?
Visually inspect the exterior vent termination for obvious blockages: snow drifts, ice buildup, bird nests, leaves, or debris. The intake and exhaust pipes should be clear with at least 12 inches of clearance from snow or obstructions. Check that the vent cap is intact and properly installed. Look for signs of poor combustion like rust staining, soot, or melted snow around the vent. Inside, inspect accessible vent sections for sagging that could trap water or condensate. If vent runs are long or have multiple bends, professional camera inspection may be needed.
What is the condensate trap and why does it cause code 31?
The condensate trap collects acidic water produced during high-efficiency combustion. It uses a U-bend design to prevent combustion gases from escaping while allowing water to drain. When the trap or drain line clogs with debris, mold, algae, or freezes, water backs up and blocks the pressure switch sensing port or disrupts airflow. This prevents the inducer motor from creating proper draft, causing code 31. Regular maintenance involves flushing the trap with bleach solution and clearing drain lines.
Can a dirty air filter cause Carrier error code 31?
Yes, a severely clogged air filter is a common cause of code 31. Restricted airflow forces the furnace to work harder, reducing the inducer motor's ability to create sufficient negative pressure for the pressure switch to close. Additionally, poor airflow can cause the heat exchanger to overheat and trigger high-limit switches, which can indirectly affect pressure switch operation. Replace filters every 1-3 months depending on use, pets, and home conditions. Check monthly during peak heating season.
How do I test the pressure switch with a multimeter?
Turn off power to the furnace. Locate the pressure switch (usually a small round device with two wires and a rubber hose). Disconnect the wires and set your multimeter to continuity or ohms mode. With no power, the switch should show infinite resistance (open). Call for heat to start the inducer motor, then gently suck on the pressure switch hose—you should hear an audible click and the multimeter should show continuity (closed circuit, near 0 ohms). If the switch doesn't click or close, it's defective. Professional testing also measures the exact pressure required to close the switch.
What is the inducer motor and why does it cause code 31?
The inducer motor is a small blower that creates negative pressure (draft) to pull combustion air through the burners and push exhaust gases through the vent system. It runs before ignition to pre-purge the heat exchanger and prove adequate draft. A weak or failing inducer motor can't create sufficient negative pressure for the pressure switch to close, triggering code 31. Signs of inducer motor failure include loud bearing noise, slow startup, excessive vibration, or visible damage to the housing or wheel. Typical lifespan is 10-15 years.
Where is the plugged metal nipple everyone mentions?
The 'infamous metal nipple' is a small threaded port on the inducer motor housing where the pressure switch sensing tube connects. Over time, carbon deposits, dust, and debris can completely clog this tiny opening, preventing the pressure switch from sensing draft even when the inducer motor runs perfectly. This is one of the most common causes of code 31 because it's small (about 1/8 inch diameter) and easily overlooked. Cleaning requires removing the hose, using a small wire or pipe cleaner to clear the port, and blowing compressed air through it.
Why does Carrier error code 31 come and go intermittently?
Intermittent code 31 indicates a marginal failure: a pressure switch near its failure threshold, partially clogged drain line that sometimes backs up, loose wire connection to the pressure switch, cracked pressure switch hose that flexes during operation, marginal inducer motor that works when cold but fails when hot, or ice formation in vent pipes during certain weather conditions. Intermittent issues are harder to diagnose because the problem may not be present when the technician arrives. Document when failures occur (time of day, weather conditions) to help diagnosis.
Can I bypass the pressure switch temporarily?
Absolutely not. Bypassing the pressure switch is extremely dangerous and illegal in most jurisdictions. The switch prevents operation when proper airflow isn't present, which could cause incomplete combustion, carbon monoxide production, overheating, cracked heat exchangers, or fire. Bypassing safety controls voids all warranties, violates building codes, creates liability issues, and puts your family at risk of carbon monoxide poisoning or death. Always address the root cause rather than defeating safety systems.
How much does it cost to fix Carrier error code 31?
Repair costs vary by root cause. Simple fixes like replacing air filters ($20-50 DIY) or clearing vent blockages ($100-200 service call) are affordable. Cleaning the condensate trap and drain lines runs $150-250. Pressure switch replacement costs $150-300 (parts + labor). Inducer motor replacement is $300-600. Complex issues like cracked heat exchangers ($1,500-3,000+) may require furnace replacement instead of repair. Always get a detailed diagnostic and written estimate before authorizing repairs. Most code 31 cases are resolved for under $300.
How long does a pressure switch last on Carrier furnaces?
Pressure switches typically last 10-15 years with proper maintenance, but can fail earlier due to water damage from condensate leaks, electrical stress from power surges, mechanical fatigue from millions of cycles, or manufacturing defects. Modern pressure switches are more reliable than older models. If your furnace is 10+ years old and experiencing code 31, pressure switch replacement is often the solution. However, always verify the switch is actually faulty rather than responding correctly to other system problems.
What should I check first when I get Carrier error code 31?
Start with these quick checks in order: (1) Replace or clean the air filter if visibly dirty. (2) Inspect exterior vent pipes for blockages (snow, debris, nests). (3) Look for standing water near the furnace indicating drain line backup. (4) Listen for inducer motor operation—it should run smoothly for 30-60 seconds before ignition. (5) Inspect visible pressure switch tubing for cracks, kinks, or disconnections. (6) If accessible, check the metal nipple connection at the inducer for debris. If all these check out and code 31 persists, professional diagnosis with proper tools is needed.
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