If your AC is running but the house isn’t cooling, the most likely culprits are a dirty air filter, low refrigerant, or a failing capacitor. None of those mean a full replacement, but they need diagnosis before the problem compounds. Here’s what I check first when a customer calls with this complaint.
Start With the Filter
Check it before you call anyone. A clogged filter restricts airflow badly enough to freeze the evaporator coil. When that happens, the unit runs, the fan blows, but barely any cold air gets through. Pull the filter. If it’s gray and matted, replace it, then run the system on “fan only” for a few hours to thaw the coil before switching back to cool.
Filters should be replaced every one to three months depending on pets, dust, and runtime. A $10 part causes a surprising number of service calls.
Frozen Evaporator Coil
If the filter looks fine but you spot ice on the copper lines near the indoor unit, the coil is frozen. Turn the system off and let it thaw fully, which can take a few hours or most of the day. You can do that part safely yourself. The question is why it froze, and that’s a different problem.
If it refreezes after you restart, there’s an underlying cause that needs a tech.
Low Refrigerant
Refrigerant doesn’t get used up under normal operation. If it’s low, there’s a leak. A system running low loses its ability to absorb heat, so the air out of the vents feels only slightly cool or close to room temperature.
Signs that point here: ice on the lines, a hissing or bubbling sound near the outdoor unit, or a system that cools fine at night but can’t keep up during the hottest afternoon hours.
This requires an EPA Section 608 certified technician. A tech will check pressures with gauges, find the leak, and repair it before recharging. Topping it off without fixing the leak means paying for the same repair twice.
Dirty Condenser Coils
The outdoor unit rejects heat to the outside. If the coils are packed with dirt, cottonwood, grass clippings, or pet hair, the system can’t shed heat efficiently. Pressures climb, cooling capacity drops, and you feel it most on hot days.
You can clear debris from around the unit and give it a gentle rinse with a garden hose. For coils that are heavily fouled or have bent fins, a tech will use the right coil cleaner and tools to restore airflow without damaging the aluminum fins.
Capacitor and Compressor Issues
The capacitor gives the compressor and fan motor the electrical jolt needed to start and run. A weak or failed capacitor is one of the most common summer failures, and it’s far cheaper to catch before it takes out the compressor.
Symptoms: the outdoor fan runs slowly or not at all, the system hums without cooling, or it trips the breaker. Capacitors hold a dangerous charge even after the power is cut. A tech tests and replaces them in a few minutes with the right equipment. Getting the microfarad rating wrong can destroy the compressor, so leave this one to a licensed tech.
If the capacitor checks out and the system still won’t cool, the compressor itself may be weak or failing. A tech can confirm that quickly once the other causes are ruled out.
Thermostat and Airflow
Worth verifying first: is the thermostat set to “cool” and not just “fan”? Fan-only blows air without cooling it. Confirm the set temperature is actually below the current room temperature.
Then walk the house and check every supply and return vent. Closed or blocked vents throw off pressure balance and cause uneven cooling. Furniture pushed against return vents is a common one.
What a Tech Does on a No-Cool Call
The diagnostic sequence is roughly:
- Check airflow and static pressure at the air handler
- Pull the filter, inspect the evaporator coil condition
- Check refrigerant pressures with a manifold gauge set
- Measure temperature split across the evaporator (a healthy system shows roughly 16-22°F between return and supply air)
- Check the outdoor unit: condenser coil condition, fan operation, capacitor readings
- Look for refrigerant leak signs at fittings and the coil
Most no-cool calls trace back to one of those. Compressor failure is less common and more expensive, but it’s diagnosable quickly once the other causes are cleared.
What You Can Safely Check First
- Replace the air filter
- Switch to fan-only mode to thaw a frozen coil
- Clear leaves, cottonwood, and debris from around the outdoor unit
- Check the circuit breaker if the outdoor unit isn’t running (once, not repeatedly)
- Confirm thermostat settings
Everything else, including refrigerant diagnosis, capacitor replacement, coil cleaning with chemicals, and anything requiring gauges or electrical work, is a job for a licensed tech.
When to Call
If the filter is clean, the settings are right, and the system still won’t cool, stop there. Running a low-refrigerant or failing-capacitor system longer pushes repair costs up and can damage other components.
We serve the Tri-Valley and East Bay. If the filter check didn’t solve it, call us and we’ll get you on the schedule fast, often same or next day when we can. Book at adriumservice.com or call us directly.