Furnace Error Code 31: What It Means & How to Fix It
Your furnace just shut down in the middle of a Michigan cold snap. The LED on the control board is flashing a sequence you have never seen before. You count the blinks: three flashes, pause, one flash. Error code 31.
Furnace error code 31 means your pressure switch is stuck in the closed position. This is one of the most common fault codes we see at NEXT Heating & Cooling during winter, especially on high-efficiency condensing furnaces. The pressure switch is a safety device that confirms proper airflow and venting before the furnace ignites. When it stays closed, the control board thinks there is a problem with combustion air or exhaust venting.
The good news: some causes of error code 31 are simple fixes. The bad news: others signal dangerous conditions like a cracked heat exchanger or blocked exhaust vent. This guide will walk you through what the code means, what causes it, and when you need a licensed Michigan HVAC technician instead of a YouTube video.
What Is Furnace Error Code 31?
The pressure switch is a small round device mounted on the furnace blower housing or near the inducer motor. It has two ports connected to rubber hoses. When the inducer motor starts, it creates negative pressure (suction) inside the heat exchanger. This pressure difference pulls a diaphragm inside the switch, closing an electrical contact. The control board detects this closed contact and knows it is safe to open the gas valve and ignite the burners.
Error code 31 means the pressure switch is already closed before the inducer motor even starts. The control board interprets this as a fault because the switch should only close when the inducer is running. If it is stuck closed, the furnace cannot verify proper venting conditions.
This is different from error codes that indicate the pressure switch failed to close (like error code 33 on many Carrier and Bryant models). Code 31 specifically means the switch is stuck in the closed position, which points to a different set of causes.
Michigan-Specific Issue: We see error code 31 spike during polar vortex events when outdoor temperatures drop below zero. Condensate drain lines freeze, exhaust vents ice over, and pressure switches fail from thermal cycling. If your furnace is in an unheated garage or poorly insulated basement, you are at higher risk.
What Causes Furnace Error Code 31
After diagnosing hundreds of furnaces across Macomb County, Oakland County, and St. Clair County, these are the most common causes of error code 31 we find:
1. Blocked Condensate Drain Line
High-efficiency furnaces produce condensation as a byproduct of combustion. This water drains through a clear plastic tube into a floor drain or condensate pump. If the drain line freezes, clogs with algae, or backs up, water accumulates in the pressure switch hose. The trapped water creates pressure that holds the switch closed even when the furnace is off.
This is the most common cause of error code 31, especially in Michigan basements where drain lines run along cold exterior walls. We have pulled ice plugs the size of a fist out of condensate lines during January service calls.
2. Failed Pressure Switch
Pressure switches wear out over time. The diaphragm inside can develop holes, the contacts can stick, or the spring mechanism can fail. Most pressure switches are rated for 100,000 to 150,000 cycles. A furnace that cycles 10 times per day will hit that limit in about 30 years, but harsh operating conditions (like Michigan humidity and temperature swings) can shorten the lifespan.
If the switch diaphragm is torn or the contacts are welded shut from electrical arcing, it will stay closed regardless of pressure conditions.
3. Cracked Heat Exchanger
This is the most dangerous cause of error code 31. A crack in the heat exchanger allows combustion gases to leak into the blower compartment. This creates positive pressure in areas where there should be negative pressure, confusing the pressure switch and causing it to close prematurely.
If you smell natural gas or a metallic odor near your furnace, or if family members are experiencing headaches or nausea, shut the furnace off immediately and call a technician. A cracked heat exchanger can leak carbon monoxide into your home.
4. Blocked Exhaust Vent
High-efficiency furnaces vent through PVC pipes that exit through the side of your house. Snow drifts, ice dams, bird nests, and even landscaping can block these vents. When the exhaust cannot escape, pressure builds up in the venting system and holds the pressure switch closed.
We have seen exhaust vents buried under three feet of snow after a lake-effect storm in St. Clair County. Homeowners do not think to check the outside vent when the furnace stops working.
5. Inducer Motor Running Continuously
If the inducer motor is stuck running (due to a failed relay or control board issue), it will keep the pressure switch closed even when the furnace is in standby mode. The control board sees the closed switch when it should be open and throws error code 31.
6. Pressure Switch Hose Problems
The rubber or silicone hoses that connect the pressure switch to the inducer housing can crack, split, or become disconnected. If a hose is pinched or has a hole, it will not transmit pressure correctly. Water, debris, or even mouse nests inside the hoses can also cause false readings.
How to Diagnose Error Code 31
Before you call a technician, there are a few safe diagnostic steps you can take. These will not fix a serious problem, but they can rule out simple issues and save you a service call fee.
Step 1: Confirm the Error Code
Find the control board inside your furnace (usually behind a metal panel on the front or side). Look for an LED light. Count the flashes. Three flashes, pause, one flash equals code 31. Write down the exact sequence. Some furnaces display different codes for similar problems.
Step 2: Check the Air Filter
A severely clogged filter restricts airflow and can cause pressure imbalances. Remove the filter. If it is packed with dust or pet hair, replace it. Turn the furnace back on. If the error clears, the filter was the problem.
Step 3: Inspect the Condensate Drain
Locate the clear plastic drain line coming from the furnace. Follow it to the floor drain or condensate pump. Look for standing water, ice, or blockages. If the line is frozen, the issue is drainage. Do not try to thaw it with a heat gun or torch. Call a technician.
Step 4: Check the Exhaust Vent Outside
Go outside and find the PVC exhaust pipe (usually white, two to three inches in diameter, often near ground level). Make sure it is not blocked by snow, ice, leaves, or debris. Clear any obstructions carefully. Do not stick anything into the pipe.
Step 5: Listen for the Inducer Motor
Turn the furnace on. Listen for the inducer motor (it sounds like a small vacuum cleaner starting up). If the motor does not start, or if it runs continuously even when the furnace is off, you have a motor or control board problem.
When to Stop: If you smell gas, hear hissing, see rust or soot around the furnace, or notice yellow flames instead of blue, stop immediately. These are signs of combustion problems or a cracked heat exchanger. Shut off the furnace and call a licensed HVAC contractor.
DIY Troubleshooting Steps (Safe Methods)
If you have confirmed the error code and ruled out obvious blockages, there are a few safe fixes you can try before calling a technician. These steps assume you are comfortable working around your furnace and have basic tools.
Clear the Condensate Drain Line
Turn off power to the furnace. Disconnect the clear drain line from the furnace (it usually pulls off with a gentle tug). Use a wet/dry vacuum to suck out any water, algae, or debris from the line. Pour a cup of white vinegar or condensate pan cleaner into the line to dissolve buildup. Reconnect the line and restore power.
Inspect Pressure Switch Hoses
Turn off power to the furnace. Locate the pressure switch (a round device about the size of a hockey puck). Check the rubber hoses connected to it. Look for cracks, splits, or disconnections. If a hose is loose, push it back onto the barbed fitting. If a hose is cracked, replace it with a new one from a hardware store (bring the old hose to match the diameter).
Reset the Furnace
Sometimes the control board gets stuck in a fault state. Turn off power to the furnace for 60 seconds. This clears the board's memory. Restore power and see if the furnace starts normally. If error code 31 returns immediately, the problem is still present.
These steps will fix about 20% of error code 31 cases in our experience. The other 80% require a technician with diagnostic tools and replacement parts.
When to Call a Michigan HVAC Technician
Some causes of error code 31 are beyond DIY repair. Here is when you need to call a licensed contractor like NEXT Heating & Cooling:
You Smell Gas or Combustion Odors
Natural gas smells like rotten eggs. If you smell it near your furnace, shut off the gas valve, open windows, and leave the house. Call your gas company from outside. Do not flip light switches or use your phone inside.
The Furnace Is More Than 15 Years Old
Older furnaces are more likely to have cracked heat exchangers. If your furnace was installed before 2010, error code 31 could indicate a serious structural problem. A technician can perform a combustion analysis and heat exchanger inspection to rule out carbon monoxide leaks.
You See Rust, Soot, or Water Stains
Rust on the heat exchanger or burners indicates long-term moisture problems. Soot around the burner compartment means incomplete combustion. Water stains on the furnace cabinet suggest a condensate leak. All of these require professional diagnosis.
The Error Code Keeps Returning
If you clear the error and it comes back within a few hours or days, the root cause is still present. Repeated cycling on error code 31 can damage the control board and igniter. A technician can test the pressure switch, measure inducer draft, and check for heat exchanger cracks.
You Hear Unusual Noises
A rumbling sound after the burners shut off suggests delayed ignition (gas buildup). A high-pitched squeal from the inducer motor means the bearings are failing. Banging or popping noises can indicate a cracked heat exchanger flexing as it heats and cools. Do not ignore these sounds.
Our NATE-certified technicians carry diagnostic tools that measure draft pressure, test pressure switch operation, and analyze combustion efficiency. We can identify a failing component before it causes a complete breakdown or safety hazard.
Cost to Fix Error Code 31 in Southeast Michigan
Repair costs depend on the underlying cause. Here is what you can expect to pay for common fixes in the Metro Detroit area as of 2026:
Pressure Switch Replacement: $200 to $400 including parts and labor. The switch itself costs $50 to $100. Labor takes about an hour. This is the most common repair for error code 31.
Condensate Drain Line Cleaning: $100 to $200 if included in a service call. If the line is frozen and requires disassembly or rerouting, add another $100 to $200.
Inducer Motor Replacement: $400 to $700 including parts and labor. Inducer motors for high-efficiency furnaces cost $250 to $450. Labor takes 1.5 to 2 hours.
Heat Exchanger Replacement: $1,200 to $2,500 depending on furnace size and brand. This is rarely cost-effective on furnaces older than 12 years. Most homeowners choose furnace replacement instead.
Control Board Replacement: $300 to $600 including parts and labor. Control boards for Carrier, Lennox, Trane, and Bryant furnaces range from $150 to $400 depending on the model.
Exhaust Vent Repair or Rerouting: $200 to $600 depending on accessibility and whether new PVC piping is required. If the vent terminates too close to a window or soffit and needs to be relocated per Michigan code, expect $400 to $800.
Most error code 31 repairs fall in the $200 to $500 range. If the diagnosis reveals a cracked heat exchanger or multiple failed components, replacement becomes the better option. A new high-efficiency furnace costs $3,500 to $6,500 installed in Southeast Michigan, depending on size, brand, and ductwork modifications.
Preventive Maintenance Saves Money: Homeowners enrolled in our Next Care Plan catch pressure switch and condensate drain issues during annual tune-ups before they cause error codes. The $5/month plan includes fall and spring inspections, priority scheduling, and 10% off repairs.
How to Prevent Furnace Error Code 31
Most error code 31 issues are preventable with routine maintenance and proper installation. Here is what we recommend to Michigan homeowners:
Schedule Annual Furnace Maintenance
A fall tune-up catches condensate drain clogs, pressure switch wear, and venting problems before they cause breakdowns. Technicians clean the condensate trap, test the pressure switch, and inspect the heat exchanger for cracks. This is the single most effective way to avoid error code 31.
Replace Air Filters Every 1 to 3 Months
Dirty filters restrict airflow and create pressure imbalances that stress the pressure switch. If you have pets, change filters monthly. Standard 1-inch filters should be replaced every 60 to 90 days. Thicker 4-inch media filters last 6 to 12 months.
Insulate Condensate Drain Lines
If your condensate drain runs along an exterior wall or through an unheated space, wrap it with foam pipe insulation. This prevents freezing during Michigan winters. We have seen drain lines freeze solid in basements that drop below 50 degrees.
Keep Exhaust Vents Clear
Check your outdoor exhaust vent after heavy snowfall. Make sure landscaping, bushes, or snow piles are not blocking the vent opening. The vent should terminate at least 12 inches above expected snow depth per Michigan mechanical code.
Install a Condensate Pump Float Switch
If your furnace drains into a condensate pump, install a float switch that shuts off the furnace if the pump fails. This prevents water backup that can trigger error code 31 and cause water damage. We install these on every furnace in basements without floor drains.
Similar to other furnace heating problems, catching pressure switch issues early prevents expensive emergency repairs during the coldest weeks of winter.
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Schedule Your ServiceFrequently Asked Questions
Yes, you can reset the furnace by turning off power for 60 seconds. This clears the control board's memory. However, if the underlying problem (stuck pressure switch, blocked drain, etc.) is still present, error code 31 will return immediately or within a few heating cycles. A reset is a diagnostic step, not a permanent fix.
Error code 31 itself is not immediately dangerous because the furnace shuts down as a safety precaution. However, the underlying cause can be dangerous. A cracked heat exchanger (one possible cause) can leak carbon monoxide into your home. If you smell gas, notice soot buildup, or experience headaches or nausea, shut off the furnace and call a technician immediately.
Most pressure switches are rated for 100,000 to 150,000 cycles, which translates to 15 to 25 years under normal operating conditions. However, Michigan's harsh winters and high humidity can shorten this lifespan. Furnaces that short-cycle frequently or operate in damp basements may need pressure switch replacement after 10 to 12 years.
Error code 31 is more common in winter because cold temperatures cause condensate drain lines to freeze and exhaust vents to ice over. High-efficiency furnaces produce more condensation in cold weather, increasing the risk of drain blockages. Additionally, furnaces cycle more frequently in winter, putting more stress on the pressure switch and increasing the chance of failure.
Yes, a severely clogged air filter can restrict airflow and create pressure imbalances that confuse the pressure switch. However, this is a less common cause of error code 31 compared to condensate drain blockages or failed pressure switches. If replacing the filter clears the error, the filter was likely overdue for replacement by several months.
If the cause is a simple fix (pressure switch replacement, drain cleaning), repair is usually the right choice. If the heat exchanger is cracked or multiple components are failing, replacement makes more sense, especially if the furnace is over 15 years old. A licensed technician can perform a cost-benefit analysis based on the furnace's age, efficiency, and repair history. Our heating services include honest assessments without upselling.
Standard homeowners insurance typically does not cover furnace repairs due to normal wear and tear or lack of maintenance. However, if error code 31 is caused by a covered peril (like a power surge from a lightning strike), your policy may cover the repair. Check with your insurance agent. Home warranty plans often cover pressure switch and inducer motor replacements if the failure is not due to neglected maintenance.