AC running but not cooling: DIY checks, repair cost, and when to call HVAC

Start with the thermostat. Is it set to "Cool," is the setpoint actually below the current room temperature, and is the fan set to "Auto" rather than "On"? A thermostat on "Fan On" will blow air constantly, which feels like the system is running even when it's not actively cooling. It sounds basic, and it's also one of the most common reasons a tech shows up and charges a diagnostic fee.

Next, look at the filter. A filter choked with dust throttles airflow across the indoor coil, drops cooling performance, and can freeze the coil into a block of ice. If you can't remember when you last changed it, change it now. Buying pleated HVAC filters in a multipack↗ that matches your return grille size is the simplest preventive move for the whole cooling season.

What a healthy AC should feel like

When the system is working right, the air at a supply register should be roughly 15 to 20°F cooler than the air entering the return. If the supply is only a few degrees cooler than the return — or feels room temperature — something is wrong. The cause is almost always airflow, refrigerant, or the compressor.

The five usual suspects

1. A clogged filter or blocked return. Cheapest, most common cause. Restricts airflow, drops capacity, and can freeze the evaporator.
2. A frozen evaporator coil. Often a downstream effect of a dirty filter, closed vents, or low refrigerant. Look for ice on the copper line at the air handler, or a puddle under it when it melts.
3. A failed run capacitor. A frequent failure on outdoor condensers, especially during a heat wave. The fan may not spin, or the compressor may hum and shut off. Affordable fix for a tech.
4. Low refrigerant from a leak. Refrigerant doesn't "get used up" in a healthy system. If it's low, there's a leak somewhere. Recharging without finding the leak is a short-term bandage.
5. A failing compressor. The expensive one. This is where repair versus replace becomes a real conversation, especially on older systems.

Homeowner-safe checks, in order

Start inside. Replace the filter. Make sure supply registers are open and returns aren't blocked by furniture or rugs. Check the drain pan under the air handler — if it's full, a safety float switch may have shut the system down. Glance at the breaker panel; a tripped AC breaker is a clue, not something to just reset repeatedly.

Then step outside to the condenser. Is it humming? Is the big fan on top spinning freely? If the unit is running but the fan isn't turning, or the fan spins when you nudge it with a stick but won't start on its own, that's a classic capacitor symptom — and that's where homeowner checks end. Capacitors hold a charge even when power is off, and opening a condenser carries both shock risk and warranty risk. Note the symptom and call.

Also check that the condenser has breathing room. Tall grass, leaves packed into the coil fins, or a shrub growing into the side will strangle performance. With the power off at the disconnect, gently rinse the coil from the outside with a garden hose, top to bottom.

Cleaning the indoor coil

If you can access the evaporator coil through a service panel on the air handler — and only with the system powered down at the breaker — a no-rinse evaporator coil cleaner↗ will cut through the film of dust that builds up over a few seasons. Spray, wait the label's dwell time, and let the condensate rinse it through the drain. If the coil is packed with matted debris or the panel is sealed, that's a job for an HVAC tech during a tune-up, not a homeowner task.

What a service call usually looks like

A standard diagnostic visit tends to run $90 to $200 depending on region and whether it's after-hours. Rough ranges for common repairs:

• Capacitor replacement: about $180 to $450.
• Contactor replacement: about $200 to $400.
• Refrigerant leak search and recharge: often $400 to $1,500+, depending on refrigerant type and whether the leak is found.
• Fan motor replacement: about $400 to $700.
• Compressor replacement: often $1,800 to $3,500+ — the point where replacing the whole outdoor unit or the full system enters the conversation.

If your system uses R-22 refrigerant, which has been phased out, recharges run expensive and the case for replacement gets stronger.

Stop and call today if…

• The outdoor unit is making a loud buzzing or grinding sound.
• The breaker trips immediately on reset.
• Ice is visible on the refrigerant lines.
• There's a burnt smell at the air handler.

Running a damaged compressor to push through a hot afternoon is how a $400 repair turns into a $3,000 one.

When replacement is the smarter spend

If the system is over 12 to 15 years old, uses R-22, and is facing a major repair — compressor, coil, or multiple simultaneous failures — putting that money into a new high-efficiency system usually comes out ahead. Newer systems also tend to dehumidify better and run quieter. Get two quotes, ask for equipment model numbers in writing, and ask about the Manual J load calculation. Oversized systems short-cycle and cool unevenly.

The goal on any hot-day AC call isn't to be impressive with the thermostat. It's to narrow the problem enough that you either solve it yourself or walk into a service call already knowing what the tech should be looking at.