Updated August 2026
Almost every service call we run traces back to a short list of causes: a clogged filter, low refrigerant, a failing thermostat, dirty condenser coils, or an electrical fault. Knowing which one you are dealing with helps you handle the easy fixes yourself, avoid the dangerous ones, and call a technician before a small problem turns into a compressor replacement. Written for Greater Seattle and Eastside homeowners, this guide covers the five most common causes of AC problems, what you can safely check on your own, and when it is time to bring in a licensed pro.
Quick Answer
The single most common cause of AC problems is a dirty or clogged air filter, which restricts airflow and drags down cooling capacity. Replace filters every one to three months and have the system professionally serviced once a year, ideally in spring, and you prevent the majority of mid-summer breakdowns.
1. Clogged Air Filters
A clogged air filter is one of the leading causes of AC problems. Over time, dust, dirt, and debris accumulate in the filter, obstructing airflow and reducing cooling efficiency. In a restricted system the evaporator coil can freeze over, the blower works harder than it was designed to, and your energy bill climbs while the house stays warm.
To prevent this issue, it is crucial to regularly clean or replace your air filters. Refer to the manufacturer’s guidelines for the recommended maintenance schedule, and check filters monthly if you have pets or seasonal allergies.
- Weak airflow from the supply vents
- Ice or frost on the indoor coil or refrigerant line
- The system runs constantly but never reaches the set temperature
- Visible dust buildup around registers
If poor indoor air quality is part of the problem, a whole-home air filtration system handles far more than a standard one-inch filter can.
For more information on the importance of clean air filters, visit the EPA’s guide on air cleaners and air filters in the home.
2. Refrigerant Leaks
Insufficient refrigerant levels can cause your AC system to blow warm air or fail to cool your space adequately. Refrigerant leaks are a common culprit behind this issue, and because refrigerant is a sealed charge rather than a consumable, a system that is low is a system that is leaking.
If you suspect a refrigerant leak, it is essential to contact a professional HVAC technician to diagnose and repair the problem. Simply topping off the charge without finding the leak wastes money and puts the compressor at risk.
⚠️ Safety and legal note: Handling refrigerant requires EPA Section 608 certification. Attempting to fix refrigerant leaks yourself can be dangerous and is not a legal DIY project.
For more information on refrigerant leaks and their impact on the environment, refer to the EPA’s guidelines on refrigerant handling requirements.
3. Faulty Thermostat
A malfunctioning thermostat can lead to irregular cooling or even a complete shutdown of your AC system. Before assuming the worst, check that the thermostat is set to cool, that the set point is actually below room temperature, and that the batteries are fresh.
If you notice any inconsistencies or suspect a faulty thermostat, consider replacing it or contacting a professional for assistance. A modern smart thermostat installation also gives you scheduling and remote control that cuts run time on mild Seattle-area days.
For tips on choosing and getting the most out of a thermostat, consult ENERGY STAR’s guide to smart thermostats.
4. Dirty Condenser Coils
Condenser coils play a crucial role in dissipating heat from your AC system. Over time, these coils can accumulate dirt, debris, and grime, hindering their ability to function efficiently. Cottonwood fluff, grass clippings, and spring pollen are the usual offenders across Greater Seattle and the Eastside.
Regularly cleaning the condenser coils can significantly improve your AC’s performance and prevent potential problems. However, it is important to follow safety guidelines and turn off the power before attempting any maintenance. Keep at least two feet of clearance around the outdoor unit and rinse the fins gently from the inside out.
To learn more about cleaning condenser coils, refer to the ENERGY STAR heating and cooling maintenance checklist.
5. Electrical Issues
Electrical problems can cause your AC system to malfunction or fail altogether. Issues like faulty wiring, tripped circuit breakers, blown fuses, or a failing capacitor can disrupt the power supply to your air conditioner. If you suspect an electrical problem, it is crucial to contact a professional HVAC technician to inspect and repair the issue safely.
⚠️ Safety note: A breaker that trips more than once is telling you something is wrong. Do not keep resetting it — shut the system off and have the circuit and equipment inspected.
For electrical safety tips and guidelines, visit the Occupational Safety and Health Administration’s electrical safety page.
Why Greater Seattle and the Eastside Are Hard on AC Systems
Cooling equipment here lives an unusual life. It sits idle from roughly October through May, then gets asked to run at full output during the handful of genuinely hot stretches we get each summer. That long dormant period is when capacitors weaken, condensate drains grow biofilm, and slow refrigerant leaks quietly bleed off charge — and none of it surfaces until the first truly hot afternoon, which is also the week every HVAC company in the region is booked solid.
A few patterns we see specifically across our service area:
- Heat pumps and ductless systems dominate. A lot of local cooling comes from a heat pump or a ductless mini split rather than a dedicated AC unit. The refrigerant and electrical failure modes are the same, but the outdoor unit works year-round, so it accumulates wear in both seasons.
- Spring pollen and cottonwood mat the coils. Outdoor units in tree-heavy neighborhoods can lose noticeable capacity by July if nobody rinses the condenser.
- Smoke season arrives right after peak heat. Late-summer wildfire smoke means the system recirculates hard while indoor air quality drops, so filtration matters more here than in much of the country.
- Cooling is often a retrofit. Plenty of older homes had AC added to equipment originally sized for heating alone, which shows up as uneven cooling and short cycling rather than an outright failure.
- Cool marine nights hide a declining system. The house recovers overnight even when daytime capacity has quietly dropped, so problems go unnoticed for years.
We cover the full Greater Seattle and Eastside service area, including Bellevue, Kirkland, and Redmond.
Homeowner AC Maintenance Schedule
Most of the problems above are preventable. This is the cadence we recommend to homeowners across Greater Seattle and the Eastside.
A maintenance plan handles the annual visit for you and keeps the tune-up on the calendar before the summer rush.
What You Can Fix vs. When to Call a Pro
Safe to Try Yourself
Replacing the air filter, changing thermostat batteries, confirming the thermostat mode and set point, clearing debris from the outdoor unit, resetting a single tripped breaker, and rinsing the condenser coil with the power off.
Call a Licensed Technician
Anything involving refrigerant, repeated breaker trips, burning smells, a frozen coil that keeps returning, loud grinding or screeching, water pooling around the air handler, or a system that will not start at all.
Don’t Wait On These
During a heat wave, an AC that runs without cooling can be a health risk for older adults, infants, and anyone with a respiratory condition. Same-day diagnosis is worth the call.
Frequently Asked Questions
Why is my AC running but not cooling?
Start with the filter and the thermostat setting, then look at the outdoor unit. A clogged filter, a dirty condenser coil, or a low refrigerant charge are the three usual explanations for a system that runs continuously without lowering the temperature.
How often should I replace my AC filter?
Every one to three months for a standard one-inch filter. Homes with pets, smokers, or allergy sufferers should check monthly. Thicker media filters can run four to six months.
Can I add refrigerant to my AC myself?
No. EPA Section 608 rules restrict refrigerant handling to certified technicians, and a system that is low on refrigerant has a leak that needs to be located and repaired rather than topped off.
Why does my AC keep tripping the breaker?
Repeated trips usually point to a hard-starting compressor, a failing capacitor, a dirty condenser coil forcing the unit to overwork, or a wiring fault. Reset it once; if it trips again, shut the system down and have it inspected.
How often should an air conditioner be serviced in the Seattle area?
Once a year, and spring is the ideal window. Servicing before the first stretch of hot weather means problems are found on a mild day instead of during the heat wave when schedules fill up fastest.
Do I really need air conditioning in the Seattle area?
More homeowners are deciding yes. Most housing here was built around heating, so upper floors can hold heat through the evening during a hot stretch. Because a heat pump provides both heating and cooling from one system, it is usually the practical way to add cooling rather than installing a summer-only unit.
Why did my AC keep up in June but struggle during a heat wave?
Capacity is rated at design conditions. A system that has lost some refrigerant charge or airflow can still hold a set point on a mild day and then fall behind once outdoor temperatures spike. It is usually an early warning rather than a sudden failure, which is why a spring tune-up catches it.
Call NW Mechanical Today
While these troubleshooting tips help with the common issues, complex problems belong with a licensed technician. Our experienced team serves Seattle, Bellevue, Kirkland, Redmond, Woodinville, and the greater Eastside, and we are well-versed in diagnosing and resolving a wide range of HVAC issues.