Trane HVAC Error Code 31: How to Fix It
Medium 1-3 hours (including thaw time) Medium Severity
Verified June 2026
- Error Code
- 31
- Brand
- Trane
- Product Type
- hvac
- Severity
- Medium
- DIY Difficulty
- Medium
- Estimated Fix Time
- 1-3 hours (including thaw time)
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Tools You'll Need
- Replacement air filter (correct size for your system)
- Flashlight or work light
- Wet/dry vacuum
- No-rinse evaporator coil cleaner spray
- White vinegar (for drain line treatment)
- Towels or absorbent rags
- Screwdriver set (Phillips and flathead)
- Multimeter (for advanced sensor testing)
How to Fix Error Code 31
-
Turn Off the System and Let It Thaw
Never use sharp objects or heat guns to remove ice from coils — this can puncture refrigerant lines and cause injury or costly damage. -
Check and Replace the Air Filter
Turn the system off before accessing the air handler or filter compartment. -
Inspect and Clear All Air Vents and Registers
-
Check the Evaporator Coil for Ice or Dirt Buildup
Always turn off power to the air handler at the circuit breaker before opening the unit cabinet. Capacitors inside can hold a dangerous electrical charge. -
Verify the Blower Motor and Fan Are Running Properly
-
Check the Thermostat Settings and Indoor Temperature
-
Inspect the Condensate Drain Line for Blockages
-
Check the Defrost Sensor or Temperature Sensor
Do not attempt to bypass or disable the anti-frost sensor. It is a critical safety component that protects your system from serious damage. -
Check for Low Refrigerant Levels
Refrigerant is a regulated substance. Only EPA-certified technicians may legally purchase, handle, and add refrigerant to HVAC systems. Do not attempt to add refrigerant yourself. -
Reset the System and Test
Parts You May Need
Air filter
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No-rinse evaporator coil cleaner
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Condensate drain pan treatment tablets
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Anti-frost temperature sensor (freeze stat)
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Blower motor
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Air filter
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No-rinse evaporator coil cleaner
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Condensate drain pan treatment tablets
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Anti-frost temperature sensor (freeze stat)
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Blower motor
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Air filter
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No-rinse evaporator coil cleaner
Check Price on Amazon
Condensate drain pan treatment tablets
Check Price on Amazon
Anti-frost temperature sensor (freeze stat)
Check Price on Amazon
Blower motor
Check Price on Amazon
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When to Call a Professional
Call a licensed HVAC technician if: the error code 31 returns repeatedly after you have cleaned the filter and cleared all vents; you see ice forming on the refrigerant lines outdoors or indoors; your system is blowing warm air even after the unit has thawed; you suspect a refrigerant leak (hissing sounds, oily residue near lines, or significantly reduced cooling); the blower motor sounds unusual or airflow is weak with a clean filter; or you need the defrost/anti-frost sensor tested and replaced. Refrigerant handling and sealed system repairs always require an EPA-certified HVAC technician.Frequently Asked Questions
What does Trane error code 31 mean?
Trane error code 31 means the system's anti-frost protection has been activated. The unit has detected that the evaporator coil or another component is at risk of freezing, so it has triggered a safety mode to prevent ice damage. This is most commonly caused by restricted airflow (dirty filter, blocked vents), a dirty evaporator coil, or low refrigerant.
Is it safe to run my Trane HVAC when it shows error code 31?
You should not continue running the compressor when error code 31 is active. Running the compressor while the coil is frozen can damage the compressor — one of the most expensive parts in the system. Switch the unit to Fan Only mode to help thaw the coil, or turn it off completely and let it thaw before troubleshooting.
How do I reset Trane error code 31?
To reset error code 31, first address the underlying cause (replace a dirty filter, clear blocked vents, clean the coil). Then turn the thermostat off and wait at least 30 seconds before turning the system back on. Simply resetting without fixing the root cause will cause the error to return quickly.
Can a dirty air filter cause Trane error code 31?
Yes — a clogged air filter is one of the most common causes of error code 31. When the filter is blocked, warm air from your home cannot flow freely across the evaporator coil. Without that warm airflow, the refrigerant inside the coil gets too cold and the coil surface temperature drops below freezing, triggering the anti-frost protection. Replacing your filter every 1 to 3 months can prevent this issue.
How much does it cost to fix Trane error code 31?
If the fix is simply replacing a dirty air filter, the cost is $5 to $30. If the coil needs professional cleaning, expect $100 to $300. If the issue is low refrigerant, a technician visit plus refrigerant recharge typically costs $200 to $600 depending on refrigerant type and amount needed. A faulty defrost sensor replacement typically runs $150 to $350 including labor. A blower motor replacement can cost $400 to $700 including parts and labor.