TRANE ERROR CODE • TRI-STATE AREA
Trane Error Code 79: Complete Troubleshooting Guide
High Pressure Lockout - Your Trane heat pump or AC detected dangerously high refrigerant pressure. Learn the causes, safe troubleshooting steps, and how to protect your compressor.
What is Trane Error Code 79?
Trane error code 79 indicates a High Pressure Lockout condition. This means the high pressure switch in your heat pump or air conditioning system detected refrigerant pressure exceeding safe operating limits—typically 400-450 PSI on the high (discharge) side of the system.
The high pressure switch is a critical safety device that protects your compressor from damage. When refrigerant pressure gets too high, the compressor must work harder, generating excessive heat that can cause mechanical failure. By shutting down before damage occurs, the system protects you from an expensive compressor replacement.
This error affects Trane heat pumps, air conditioners, and packaged units. On communicating systems with digital displays, you'll see error code 79. On non-communicating units, the outdoor unit may simply not run, with the high pressure switch in the open position.
Protect Your Investment
Repeatedly resetting a system with high pressure issues can destroy your compressor. If code 79 returns after one reset, stop using the system until it's professionally diagnosed. Compressor replacement is one of the most expensive HVAC repairs—far more than the repair that would prevent it.
Common Causes of Trane Error Code 79
High refrigerant pressure results from the system's inability to release heat effectively or from improper refrigerant levels. Understanding these causes helps determine if you can troubleshoot safely or need professional service:
1Dirty Condenser Coils
The most common cause of code 79. Condenser coils release heat from refrigerant to outdoor air. When coated with dirt, grass clippings, cottonwood seeds, or debris, heat transfer is impaired. The system can't reject enough heat, causing pressure to rise. This is the first thing to check and often the easiest to fix with a thorough cleaning.
2Condenser Fan Motor Failure
The condenser fan pulls air across the coils to remove heat. If the fan motor fails, runs slowly due to a bad capacitor, or has broken blades, airflow drops drastically. Without adequate airflow, even clean coils can't release heat effectively. Listen for the fan when your system runs—silence from the outdoor unit indicates a fan problem.
3Blocked Airflow Around Outdoor Unit
The outdoor unit needs at least 24 inches of clearance on all sides for proper airflow. Plants, fences, storage items, or debris piled against the unit restrict air movement. Even if coils are clean, blocked airflow causes the same heat rejection problems. Ensure your unit has proper clearance on all sides.
4Refrigerant Overcharge
Too much refrigerant in the system directly causes high pressure. This typically happens after improper service work where refrigerant was added without proper diagnosis. Symptoms include high pressure on both high and low sides, poor cooling performance, and higher-than-normal energy bills. Only a professional can properly recover excess refrigerant.
5Metering Device (TXV) Malfunction
The thermostatic expansion valve (TXV) controls refrigerant flow into the evaporator. If it fails in a restricted position, refrigerant backs up in the high side, causing elevated pressure. TXV issues typically also cause poor cooling performance and potential evaporator icing. This requires professional diagnosis and replacement.
6Non-Condensables in System
Air or other gases trapped in the refrigerant system don't condense like refrigerant and take up space in the condenser. This effectively reduces condenser capacity and raises pressure. Non-condensables enter during improper service or from leaks. Proper evacuation and recharging by a professional resolves this issue.
DIY Troubleshooting Steps for Error Code 79
Several causes of code 79 can be checked and corrected without professional help. Follow these steps before calling for service:
1Turn Off the System and Wait
Set your thermostat to OFF and wait at least 5-10 minutes. This allows refrigerant pressure to equalize and gives the high pressure switch time to reset. Never attempt to bypass or reset the high pressure switch manually—it's a critical safety device.
2Inspect and Clean Condenser Coils
Turn off power to the outdoor unit at the disconnect box. Remove any visible debris (leaves, grass, cottonwood seeds) from around and on the coils. Use a garden hose to gently spray the coils from inside out—never use a pressure washer as it damages fins. For heavy buildup, use a commercial coil cleaner following product directions.
Tip: Clean coils in spring before cooling season and in fall before heating season (for heat pumps). Monthly visual inspections during heavy use periods help prevent buildup.
3Clear Obstructions Around the Unit
Ensure at least 24 inches of clearance on all sides of the outdoor unit. Trim back plants, move storage items, and remove any debris. Check above the unit too—overhanging branches or structures can impede heat rise. The unit needs unrestricted airflow to operate properly.
4Check the Condenser Fan
With power off, try spinning the fan blade by hand. It should spin freely. If it's stiff or grinding, the motor bearings may be failing. With power restored, observe whether the fan runs when the system starts. If the compressor runs but the fan doesn't, you've found the problem—this requires professional repair.
5Check Indoor Air Filter
A severely clogged indoor air filter restricts airflow across the evaporator coil. While this more commonly causes low pressure issues, it can contribute to overall system strain. Replace your filter if dirty—it's quick, inexpensive, and always beneficial for system operation.
6Attempt a Restart
After cleaning coils and clearing obstructions, restore power and set the thermostat to call for cooling (or heating if using heat pump). The system should start normally. Monitor it for 15-20 minutes—if it runs without tripping again, your troubleshooting was successful.
Important: If code 79 returns immediately or within the first cooling cycle, stop using the system. The issue requires professional diagnosis—continued operation risks compressor damage.
Professional Repair for Trane Error Code 79
Common Repairs
- Condenser Fan Motor: Restores proper heat dissipation
- Fan Capacitor: Quick fix for slow or non-running fans
- TXV Replacement: Corrects refrigerant metering issues
- Refrigerant Adjustment: Proper charge for optimal performance
- Professional Coil Cleaning: Maximizes heat transfer efficiency
Why Professional Service Matters
- Proper pressure diagnosis with gauges
- Refrigerant handling certification required
- Identify root cause vs. symptoms
- Prevent compressor damage
- Warranty-compliant repairs
Expert Trane Service in the Tri-State Area
Our technicians carry pressure gauges, refrigerant recovery equipment, and common replacement parts. Most code 79 issues are resolved same-day.
Preventing Trane Error Code 79
Regular Coil Cleaning
Clean condenser coils at least twice per year—spring and fall. Monthly visual inspections during cooling season catch debris buildup early before it causes problems.
Maintain Clearance
Keep 24 inches clear around the outdoor unit at all times. Regularly trim plants and avoid using the area for storage. Good airflow prevents high pressure conditions.
Annual Professional Maintenance
Schedule professional AC maintenance annually. Technicians check refrigerant charge, clean coils thoroughly, test capacitors, and verify proper operation.
Change Indoor Filters
Replace filters every 30-90 days. Proper indoor airflow helps the entire system operate efficiently and reduces strain on outdoor components.
Frequently Asked Questions About Trane Error Code 79
What is Trane error code 79?
Trane error code 79 indicates a High Pressure Lockout condition. This means your heat pump or air conditioning system detected refrigerant pressure exceeding safe operating limits, typically above 400-450 PSI. The system shuts down to prevent compressor damage and won't restart until the pressure normalizes and the system is reset.
What causes high pressure in a Trane heat pump?
Common causes include dirty condenser coils restricting heat release, a failed condenser fan motor, blocked airflow around the outdoor unit, refrigerant overcharge, a malfunctioning metering device (TXV), or extremely high outdoor temperatures combined with heavy system demand.
Can I reset Trane error code 79 myself?
You can attempt a reset by turning off the system at the thermostat, waiting 5 minutes for pressures to equalize, then turning it back on. However, if the high pressure condition was caused by a system problem rather than a temporary spike, the code will return. Address underlying causes first.
Is Trane error code 79 dangerous?
The lockout itself is a safety feature protecting your compressor from damage due to excessive pressure. Operating a system with high pressure can cause compressor failure or refrigerant line ruptures. The system is designed to shut down before damage occurs.
Can dirty coils cause Trane error code 79?
Yes, dirty condenser coils are one of the most common causes of high pressure lockouts. When coils are coated with dirt, grass, cottonwood seeds, or debris, they can't release heat effectively. This causes refrigerant pressure to rise until the high pressure switch triggers.
How do I clean my Trane condenser coils?
Turn off power to the unit, remove debris from around the unit, use a garden hose to spray coils from inside out (not a pressure washer), and use coil cleaner for stubborn buildup. Professional cleaning during annual maintenance ensures thorough results without damaging fins.
Why does my Trane unit get high pressure in hot weather?
Heat pumps and AC systems work harder when outdoor temperatures are extreme. Combined with high indoor cooling demand, this can push pressures near limits. If your system only trips in very hot weather, ensure coils are clean and airflow is unrestricted. Systems in good condition should handle normal summer temperatures.
Can too much refrigerant cause code 79?
Yes, overcharging the system with refrigerant directly causes high pressure. This can happen after improper service work or when refrigerant is added without proper diagnosis. Overcharge requires a professional to recover excess refrigerant and properly adjust the charge.
What is a TXV and can it cause code 79?
The TXV (Thermostatic Expansion Valve) regulates refrigerant flow into the evaporator. If it fails partially closed, it restricts flow and causes high head pressure. TXV issues require professional diagnosis and replacement—this is not a DIY repair.
How long does it take to fix Trane error code 79?
Repair time depends on the cause. Coil cleaning takes about an hour. Condenser fan motor replacement typically takes 1-2 hours. TXV replacement may require 2-3 hours. Most repairs can be completed in a single service visit when parts are available.
Should I run my Trane system after code 79?
You can try restarting after the system cools (wait at least 5 minutes), but repeated high pressure events cause compressor wear. If the code returns, stop using the system until it's professionally diagnosed to prevent expensive compressor damage.
Can a bad capacitor cause high pressure in Trane units?
Yes, a failing run capacitor for the condenser fan motor can cause the fan to run slowly or not at all. Without proper fan operation, the condenser can't reject heat, causing pressure to spike. Capacitor testing and replacement is a common repair.
What's the difference between code 79 and code 126?
Code 79 is High Pressure Lockout (cooling/heat pump system pressure issue). Code 126 is Ignition Lockout (furnace/heating ignition issue). They affect completely different components—79 relates to refrigerant pressure, 126 relates to gas burner ignition.
Can blocked return vents cause code 79?
Indirectly, yes. Severely restricted indoor airflow reduces heat absorption by the evaporator, which can cause the system to work harder and contribute to high pressure. Ensure all supply and return vents are open and unobstructed.
Who repairs Trane heat pumps with code 79 in the tri-state area?
Dimatic Control LLC provides expert Trane heat pump and AC repair throughout the tri-state area. Our technicians carry diagnostic equipment to measure refrigerant pressures and identify high pressure causes. Call (908) 249-9701 for same-day service.
Need Help with Trane Error Code 79?
Don't risk compressor damage from repeated high pressure events. Our certified technicians diagnose and repair Trane systems throughout the tri-state area.
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