Introduction
There’s nothing more frustrating than coming home on a hot summer day, only to discover that your air conditioner is not cooling. While many people assume the worst and call a technician, you might be surprised to learn that you can fix many common AC issues yourself. In this 2025 DIY repair guide, you’ll learn step-by-step how to fix an air conditioner that’s not cooling, including diagnostics, solutions, maintenance tips, and when to call a professional.
Understanding How Your Air Conditioner Works
Before diving into fixes, it’s helpful to understand the basic components of a home AC unit:
- Evaporator Coil: Absorbs heat from the air inside your home.
- Compressor: Pressurizes refrigerant and circulates it.
- Condenser Coil: Releases heat outdoors.
- Fan and Blower: Move air across coils and into your rooms.
- Thermostat: Controls temperature and cycles the system on/off.
When any of these components fail or underperform, your AC may stop cooling effectively.
Step-by-Step: How to Fix an Air Conditioner That’s Not Cooling
Step 1: Check the Thermostat Settings
Start with the basics:
- Make sure your thermostat is set to “Cool” mode.
- Lower the temperature 5 degrees below current room temperature.
- If you have a programmable or smart thermostat, ensure it’s not in vacation or eco mode.
Tip: Replace the thermostat batteries if it’s not responding or has a blank screen.
Step 2: Inspect the Air Filter
A clogged air filter restricts airflow, causing your AC to struggle or stop cooling altogether.
- Locate the air filter (usually near the return duct or air handler).
- If it’s covered in dust, replace or clean it.
- Use high-efficiency filters rated MERV 8 to 13 for best results.
Recommendation: Replace filters every 1–2 months during high-use seasons.
Step 3: Ensure the Outdoor Unit Has Power
If the outdoor condenser unit isn’t running, the AC won’t cool.
- Check your circuit breaker and reset it if tripped.
- Make sure the outdoor disconnect switch is on.
- Listen for humming or fan noise outside—if there’s none, power might be the issue.
Step 4: Clear Debris from the Outdoor Unit
Leaves, dirt, and debris can block airflow and reduce cooling.
- Turn off power to the unit.
- Remove any visible debris from the unit’s exterior.
- Use a hose (low pressure) to clean the condenser fins.
- Straighten bent fins using a fin comb.
Caution: Don’t use a pressure washer—it can damage the coils.
Step 5: Examine the Refrigerant Lines for Ice
Frozen refrigerant lines or evaporator coils signal airflow issues or low refrigerant.
- Turn off the unit and let it thaw for 4–6 hours.
- Inspect for frost or ice build-up on copper lines.
- After thawing, turn on the fan-only mode to dry out the system.
If freezing recurs, contact a professional—it may require a refrigerant recharge.
Step 6: Clean the Evaporator Coil
A dirty coil can’t absorb heat properly.
- Turn off power to the indoor unit.
- Open the access panel.
- Use a soft brush or coil cleaner spray to clean the coil surface.
- Dry it with a clean cloth before replacing the panel.
Step 7: Check for Leaky Ductwork
Poorly sealed ducts leak cool air into the attic or crawlspace.
- Inspect accessible ducts for gaps, holes, or loose joints.
- Seal with metal-backed duct tape (not cloth duct tape).
- Consider a professional duct inspection for hidden leaks.
Step 8: Test the Capacitors and Contactor
Capacitors start the compressor and fan. The contactor switches power to the system.
- Remove the outdoor unit cover (turn off power first).
- Use a multimeter to test the run/start capacitor and contactor.
- If they show signs of bulging, leaking, or no charge, replace them.
Warning: Capacitors store high voltage. Discharge them before touching.
Step 9: Check the Drain Line for Clogs
A clogged drain line can cause the system to shut off or freeze.
- Locate the PVC condensate drain line near your air handler.
- Use a wet/dry vacuum to suction the line from outside.
- Flush with vinegar every 2–3 months as preventive maintenance.
Step 10: Evaluate the Compressor Function
The compressor is the heart of your AC. If it’s not working:
- You may hear a loud hum, click, or nothing at all.
- Test with a multimeter or listen for fan vs compressor noise.
- If the fan runs but the compressor doesn’t, the start relay or capacitor may be faulty.
Note: Compressor repairs can be expensive—compare repair vs. replacement costs.
Advanced Troubleshooting
- Low Refrigerant: Requires a licensed technician to diagnose and recharge.
- Dirty Blower Motor: Clean blades and ensure it’s spinning freely.
- Faulty Expansion Valve or TXV: Causes poor cooling; not DIY-fixable.
- Software/Firmware Issues in Smart Thermostats: Check manufacturer support.
Preventive Maintenance Tips
- Schedule annual AC tune-ups every spring.
- Keep shrubs and grass trimmed around the outdoor unit.
- Change filters regularly.
- Use a surge protector for your HVAC system.
- Keep supply and return vents clear and unblocked.
When to Call a Professional
- Your system is over 10–12 years old.
- Compressor or refrigerant problems.
- Electrical components test faulty and you’re not experienced.
- You’ve tried DIY fixes and AC still isn’t cooling.
Estimated Costs of Common Repairs
- Air filter replacement: $10–$30
- Capacitor: $15–$50 DIY / $120 with labor
- Refrigerant recharge: $150–$500 (requires license)
- New compressor: $700–$1,500
- Full AC unit replacement: $3,500–$7,000
Applicable Brands
These steps apply to most major residential AC brands:
- Carrier
- Lennox
- Goodman
- Trane
- Rheem
- American Standard
- York
- Bryant
FAQ – Fixing AC Not Cooling
Q1: Why is my AC running but not cooling?
A: Likely a clogged filter, low refrigerant, or dirty coils.
Q2: How do I reset my AC unit?
A: Turn off the system at the thermostat and breaker for 5 minutes, then restart.
Q3: Is low refrigerant a DIY repair?
A: No. It requires licensed HVAC technicians with EPA certification.
Q4: What’s the best temperature to set AC in summer?
A: 24–26°C (75–78°F) for comfort and efficiency.
Q5: Why is my outdoor fan spinning but indoor unit blowing warm air?
A: Could be frozen coils, low refrigerant, or failed compressor.
Conclusion
Learning how to fix an air conditioner that’s not cooling can save you time and money—especially during the heat of summer. Start with simple fixes like checking the thermostat, replacing filters, and clearing debris. With the right tools and a careful approach, many issues can be resolved at home. Always prioritize safety, and don’t hesitate to call a pro for electrical or refrigerant-related problems.
