The Complete HVAC Troubleshooting Guide
It’s the hottest day of the year, and your AC stops working — a truly broken AC situation. Or your furnace dies in January. You know the feeling. Here’s something most homeowners don’t realize: plenty of common AC problems have fixes so simple you can handle them yourself. Service calls often start at $100 just to get someone to your door. A tripped breaker or dirty filter? You’ve just paid someone to flip a switch.
Learning basic troubleshooting AC skills saves money and time waiting around. This air conditioner troubleshooting guide covers the problems we see most often, what’s causing them, and what you can do yourself. Some situations do need a professional, and we’ll explain how to recognize them.
Call Chill Heating & Cooling today for expert HVAC repair in your area
What Is an HVAC System and What Does It Do?
HVAC is short for Heating, Ventilation, and Air Conditioning. An HVAC system has three main functions: control temperature, manage humidity, and filter air quality. It pulls air from your house, heats or cools it, then pushes it back through vents.
How Does an HVAC System Work?
Key components:
- Thermostat – Command center that tells your system when to heat or cool
- Air handler – Houses the blower, circulating air through ductwork
- Evaporator coil – Cools air by absorbing heat inside the air handler
- Condenser unit – Outdoor box that dumps heat outside
- Compressor – This component moves refrigerant between your indoor and outdoor units
- Refrigerant lines – These copper pipes carry refrigerant and move heat around
- Ductwork – Delivers conditioned air to every room
- Air filter – Stops dust and debris from getting into your system
The cooling process works like this: refrigerant soaks up heat from inside, moves outside where it dumps that heat, then loops back inside to repeat. If your air conditioner is not cooling, something has gone wrong in this process. While troubleshooting AC problems isn’t for everyone, it can save you time, money, and comfort.
10 Common AC Problems and How to Fix Them
These are the top 10 AC problems we run into. Some are easier than others.
1. AC Won’t Turn On? Here’s What to Check First
You flip the thermostat to cool. Nothing happens. Total silence. You suspect a broken AC unit.
Solution:
Air conditioner not cooling? Start with your thermostat. Confirm it’s set to “cool” and the target temp is lower than what the room actually is. Battery-powered models fail all the time when batteries die. Remove the cover and replace the batteries.
Next stop is your circuit breaker. Most AC units have their own breaker marked “AC” or “Air Handler.” Tripped? Push it all the way to off first, then back on. Look at the disconnect box outside near your condenser for blown fuses. There’s also a power switch near your indoor unit that looks like a light switch. People accidentally flip it off more often than you’d guess.
Check your condensate drain pan too. When water backs up there, a safety switch kills power to prevent flooding. Empty the pan.
Still dead? Failed capacitor, busted contactor, or wiring issue. Time for AC repair.
2. AC Keeps Turning On and Off (Short Cycling)
Runs for a bit. Stops. Starts again. Over and over.
This wrecks your compressor and wastes energy. Try these AC unit troubleshooting tips.
Solution:
Check your air filter. Can’t see through it? There’s your problem. A choked filter makes the system overheat and shut down.
Look where your thermostat is. Lamp, TV, or sunny window nearby? It’s getting false readings. The broken AC thinks it’s hotter than it is and cycles constantly. Move the heat source.
Head outside and clear debris away from your condenser. Gently rinse the outdoor coils with a garden hose. See ice forming on those copper refrigerant lines? That points to low refrigerant or airflow problems. Shut everything down and let it thaw completely.
Your AC might just be too big for your space. Oversized units cool too fast. None of this is working? It could be refrigerant leaks, a failing compressor, or electrical problems. Get in touch with residential air conditioning services to fix your broken AC unit.
3. AC Runs Constantly and Never Shuts Off
Just keep going and your AC is not cooling. Never hit the temperature you set; the AC is running but not cooling.
Your electric bill’s going to hurt. Troubleshooting AC problems like this is fairly simple.
Solution:
Check the thermostat mode. “Fan on” keeps the blower going constantly, even between cooling cycles. Switch it to auto.
On brutal hot days, your AC might struggle if you’ve set it too low. Bump it up a few degrees and wait an hour. Dirty filter? Replace it. Air escaping through gaps? Close them up with weatherstripping. Outdoor unit sitting in full sun all day? That makes it work much harder. Add some shade.
Sometimes the system’s just too small for the house. Or you’ve got refrigerant leaks since your AC is not cooling right, despite running nonstop. Call for AC maintenance to check levels.
4. AC Is On but Not Blowing Cold Air
You hear it running. Air’s coming out. Feels lukewarm though; your AC is not blowing cold air, but is running.
Solution:
Look at the thermostat mode. Should be set to “cool,” not “fan only.” Fan mode moves air around without actually cooling it.
Dirty filter tops the list for AC not blowing cold air. Swap it. Should be changed every 1-3 months anyway.
Walk outside. Is the condenser fan spinning? Compressor humming, but the fan is not moving? Usually, it’s a dead capacitor or fan motor.
Hit the outdoor coils with a hose if they’re caked with dirt. Dirty coils lose their ability to transfer heat. Result? An AC is not blowing cold air, but is running.
Ice on the refrigerant lines or indoor coils? Shut down and let it thaw. Is ice returning after you restart? You’ve got airflow or refrigerant problems. Low refrigerant indicates a leak somewhere.
Still warm? Get emergency air conditioner repair if your AC is not cooling or residential heat pump repair.
Ready to fix your HVAC system the right way? Let our certified technicians handle the problem
5. Weak or No Airflow from Vents
Barely any air is coming out of your vents, and some rooms get zero.
Solution:
Every vent open? Furniture and curtains block more than you’d think. Open all registers.
Clogged filters are usually the culprit. This usually takes only a minute or two. Also, check your ductwork in the attic or basement.
Disconnected sections, holes, or crushed ducts? Those leaks waste up to 30% of your cooled air. Seal small gaps with metal tape. Regular duct tape falls apart too quickly.
Return vents matter just as much. The system needs to pull air in to push it back out. Don’t block them. And closing vents in unused rooms actually hurts your system. Modern HVAC is balanced for the whole house.
Flow still weak? Ducts might be undersized, or the blower motor might be dying. Call for heating services.
6. Water or Refrigerant Leaking from Your AC
Puddle near your indoor unit or liquid dripping from the condenser.
Solution:
Two different leaks. Water’s common and usually simple. Refrigerant leaks require a professional; it is likely your AC system is not working.
Water leaks? Nine times out of ten, it’s a clogged condensate drain line. Find the drain line near your indoor unit. There’s a PVC pipe running somewhere. Pour a cup of bleach or vinegar into it, give it half an hour, then flush with water. That clears most algae clogs.
Check the condensate pan under your indoor unit. Rust holes or cracks let water spill everywhere. The drain line also needs to slope downward along its entire length or water won’t drain properly.
Refrigerant leaks look different. Oily residue on the lines, sweet chemical smell, ice on copper pipes. Leave it alone. Refrigerant’s toxic. Plus, low refrigerant means your AC system is not working at full power.
Refrigerant leak suspected or your AC system is not working? Call for residential air conditioning services.
7. AC Has Drainage Problems or a Clogged Drain Line
Water is backing up, or there are wet spots around your indoor unit. Try our AC unit troubleshooting tips.
Solution:
Find your drain pan under the air handler. Water sitting there means the line’s clogged. Track where the line exits your house. Take a wet/dry vacuum and stick the hose over the opening. Run it for a few minutes to pull out most clogs. Or use a drain snake from the access point.
There might be a float switch in the pan. Water rises too high, and the switch cuts power. Once the clog is cleared, the switch should reset automatically as water levels drop.
Vinegar poured down the line every few months stops future clogs. Drain keeps clogging anyway? The line probably has the wrong pitch. Get AC maintenance if this becomes a pattern.
8. Strange or Musty Smells Coming from the AC
Musty, moldy, burning smells are blowing through your house.
Solution:
Musty smell? Moisture and mildew are somewhere in the system. Shut it down and change the filter. If you can get to your evaporator coils, spray them with coil cleaner. Scrub the drain pan with soap and water, then dump bleach down the drain line.
Burning smells are serious. Points to electrical problems or overheating parts. Burning plastic smell? Shut the system down immediately. Don’t restart it until a tech examines the electrical components.
Chemical smell? Refrigerant leak. Turn it off and open the windows. Smell something dead? Probably a rodent or bird that got into your ductwork. Unpleasant job, but you’ll have to locate and remove it.
Do smells stick around? Call for our residential air conditioning services right away.
9. Loud or Unusual Noises from Your AC
Banging. Grinding. Squealing. Rattling. Normal operation is quiet.
Solution:
Rattling usually means loose parts or debris. Check your outdoor unit for sticks hitting the fan. Tighten the top panel if it’s vibrating. Squealing? Belt problems or bearing issues in the blower motor.
Grinding’s bad. Often, it means your compressor or motor bearings are failing. Shut it down immediately. Continuing to run a grinding motor can destroy the unit.
Banging happens when the fan blade hits something. Kill the power. Clicking at startup’s fine. Constant clicking means a relay is dying. Buzzing comes from electrical stuff. Loose wiring, failing contactor, bad capacitor. Hissing might be a refrigerant leak.
Don’t ignore weird noises. Get an AC repair or residential heat pump maintenance scheduled.
10. Thermostat Not Working or Giving Wrong Readings
Shows the wrong temperature, won’t respond, or the screen is blank.
Solution:
Battery-powered? Check those first. Pop the faceplate off and blow out dust with compressed air. Dirt interferes with sensors.
The thermostat needs to sit level. Test its accuracy by sticking a separate thermometer next to it. Wait 15 minutes and compare. Off by more than a few degrees? Time for recalibration or replacement.
Kill power at the breaker, pull off the faceplate, and check for loose wires. Location matters too. Near heat sources, drafts, or direct sunlight?
Can’t read room temp accurately.
Old thermostats die. Most last about 10 years. Still having issues after checking these things? Your system’s control board might be the problem, and you need more air conditioner troubleshooting. Call Chill Heating & Cooling for everything from AC repair to residential heat pump installation.
Maintenance Tips to Keep Your HVAC Healthy
Simple routine tasks prevent the most common AC problems.
- Change filters every 1-3 months – Dirty filters cause nearly half of all air conditioning issues
- Keep outdoor units clear – Remove debris within two feet of the condenser
- Clean vents – Vacuum dust buildup from registers
- Check condensate drain monthly – Pour vinegar down the line
- Inspect refrigerant lines twice a year – Look for damaged insulation
- Schedule professional maintenance annually – Techs catch small problems early
- Test your thermostat seasonally – Verify it responds correctly
- Listen for unusual sounds – Catching noise problems early prevents failures
- Monitor energy bills – Sudden spikes indicate problems
- Clear drain pans – Check for standing water
Professional AC maintenance spots air conditioning issues you’d miss and keeps your warranty intact.
When to Call an HVAC Professional
Some air conditioning issues need trained technicians with specialized tools, even after going through our full air conditioner troubleshooting guide.
- Refrigerant leaks or low levels – Only licensed techs can legally work with refrigerant
- Electrical problems or frequent breaker trips – Faulty wiring is a serious fire hazard
- Compressor or motor failures – These expensive parts require expert replacement
- Ductwork damage or improper sizing – Leaking ducts waste tons of energy
- Ice buildup that returns after thawing – Repeated ice formation points to deeper issues
- System short cycles despite troubleshooting – Ongoing cycling indicates mechanical failures
- Complete system failures – If your troubleshooting AC efforts don’t restore power, call for help
- Strange smells that persist – Ongoing odors might indicate dangerous issues
- Water damage or flooding – Major drainage problems require immediate attention
- Annual maintenance and tune-ups – Pro servicing protects your warranty
- AC replacement – If your air conditioner is on its last leg, you need a pro’s help.
Chill Heating & Cooling provides same-day service for emergency air conditioner repair throughout the Bay Area. We’ve seen all types of air conditioning problems and solutions.
Beat HVAC Headaches: Conclusion & Next Steps
Most air conditioning problems and solutions start with the basics. Change filters, check breakers, and clear drain lines. Spot these common AC problems early and you’ll avoid expensive repairs down the road. Regular maintenance matters, and so does fixing problems the moment you notice them. Worked through these air conditioner troubleshooting steps already? Still having issues? Need residential heat pump services, or don’t have time for troubleshooting AC units? Chill Heating & Cooling provides diagnosis and repair in San Ramon and surrounding areas. Call (925) 350-7558 for same-day service.
Talk to a trusted HVAC technician now and get your system running smoothly again!
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