AC Not Cooling? A Complete Troubleshooting Guide

Your air conditioner is running but the house won’t cool down. Or it’s not turning on at all. Before calling an HVAC tech — or after your tech visit reveals nothing obvious — this guide walks through the most likely causes, organized by what you can check yourself and what requires a professional.

Many of the most common AC cooling failures have simple fixes. Others are signs of a bigger problem. Knowing the difference saves time and money.

Step One: Basic Checks Before Anything Else

Start with the obvious before diving deeper. These take under five minutes.

Check your thermostat settings. Confirm it’s set to COOL, not FAN or HEAT. Set the target temperature at least 3–5 degrees below the current room temp. If it’s battery-powered, replace the batteries.

Check your air filter. A clogged filter restricts airflow so severely that the system can’t cool effectively — and in extreme cases, causes the evaporator coil to freeze. Slide out the filter. If it’s gray and matted with dust, replace it now. This alone fixes a surprising number of “not cooling” calls.

Check your circuit breakers. The AC compressor and the air handler/furnace may be on separate breakers. Check both. A tripped breaker looks centered between ON and OFF. Reset by pushing fully to OFF first, then back to ON.

Check that all vents are open. Closing too many supply vents increases static pressure and reduces system efficiency. Make sure at least 80% of vents are fully open.

If all of these check out, keep reading.

Thermostat Problems

The thermostat is the brain. When it malfunctions, the system can’t get the right commands.

Symptoms: AC runs constantly but won’t reach setpoint; AC won’t turn on at all; temperature reading seems inaccurate

DIY checks:

Confirm the display is on and reading correctly
Check for loose wiring at the thermostat (turn off power first)
Make sure the thermostat isn’t in direct sunlight or near a lamp, which gives it a false high reading
Try a manual override or “cool on” mode

Smart thermostat issues: If you recently installed a smart thermostat, check that the C-wire (common wire) is properly connected. Smart thermostats need continuous power; without a C-wire they may power-steal from other terminals, causing erratic behavior.

When to call: Wiring issues beyond your comfort level, thermostat that reads correctly but doesn’t communicate with the system

Replacement cost: Programmable thermostat $30–$80 DIY; smart thermostat $150–$350 installed by a pro

Dirty or Frozen Evaporator Coil

The evaporator coil lives inside your air handler. Refrigerant flows through it and absorbs heat from your air. When it gets dirty or freezes, heat transfer stops.

Symptoms: Warm air from vents, ice visible on refrigerant lines, ice on the indoor unit, water dripping from air handler

Why it freezes:

Dirty air filter (most common)
Low refrigerant (creates abnormally low pressure, drops coil temp below freezing)
Dirty evaporator coil surface
Blower motor running slow or failing

DIY fix for a frozen coil:

Turn system to FAN ONLY (not cooling) for 2–4 hours — or shut it off entirely
Never chip at the ice; it will damage the coil fins
Once thawed, change the filter and run again
If it refreezes, the cause is refrigerant or equipment failure — call a tech

Dirty coil: This is not a DIY repair. Coil cleaning requires accessing the air handler, applying chemical cleaner, and rinsing — the chemicals and process require training and proper disposal. Expect $100–$300 for a professional coil cleaning.

Refrigerant Issues

Refrigerant doesn’t get “used up” like fuel. If levels are low, there’s a leak somewhere. Low refrigerant is one of the most common reasons an AC runs but won’t cool.

Symptoms:

AC runs all day but house stays 5–10°F above setpoint
Ice on the refrigerant lines near the outdoor unit
Hissing or bubbling sound near the unit
System was recharged before but needed it again

What refrigerant does: It cycles between your indoor and outdoor units, absorbing heat inside your home and releasing it outside. With insufficient refrigerant, the system can’t transfer enough heat.

DIY: None. Handling refrigerant requires EPA Section 608 certification. Adding refrigerant without fixing the leak is also just a temporary band-aid.

What a tech will do:

Check pressures with manifold gauges
Test for leaks (electronic detector, UV dye)
Repair the leak
Recharge to manufacturer specifications

Cost: $200–$600 for leak detection and recharge. If the leak is in the coil, repair gets expensive — a leaking evaporator coil replacement can run $1,500–$2,800.

R-22 note: Systems manufactured before ~2010 use R-22 (Freon), which is no longer produced in the US. R-22 recharge runs $50–$150/lb and is getting scarce. If your R-22 system has a significant leak, replacement of the whole system often makes more financial sense.

Outdoor Condenser Unit Problems

The condenser is the outdoor unit — the one that blows hot air out. It does the other half of the heat transfer job: rejecting heat from the refrigerant to outside air. When it can’t work properly, the whole system suffers.

Dirty condenser coils

The condenser coils on the outside of the unit collect dirt, cottonwood fluff, grass clippings, and debris. When caked, they can’t release heat.

DIY: Shut off power at the disconnect box. Use a garden hose to gently spray outward through the fins (not inward — you’ll push debris further in). Avoid pressure washers on the fins.

Blocked airflow

The condenser needs clearance. Check for:

Overgrown shrubs or vines (keep 18–24 inches of clearance)
Debris like leaves packed against the unit
Anything placed on top of the unit

Condenser fan motor failure

The condenser fan pulls air over the coils. If it fails, heat can’t escape — high-pressure cutouts trip and the system shuts down.

Symptoms: Outdoor unit makes noise but fan blade isn’t spinning; unit cycles off quickly and won’t restart

Cost: $200–$500 for fan motor replacement

Compressor failure

The compressor is the heart of the system. It pressurizes refrigerant to drive the heat exchange cycle. Compressor failure usually means the system won’t cool at all.

Symptoms: Outdoor unit hums but compressor doesn’t start; loud grinding or screeching; hard start kicks and trips

Cost: $800–$2,500 for compressor replacement. On an older system, this expense often tips the calculation toward full replacement.

Ductwork Problems

Your AC may be cooling properly but leaky or disconnected ducts dump that cool air into unconditioned space before it reaches your rooms.

Symptoms:

Some rooms cool much better than others
High energy bills without matching comfort
Dusty air or visible gaps in duct connections in attic or crawlspace

DIY check: Feel along accessible duct runs in the attic or basement for air leaking at connections. Look for disconnected sections.

DIY fix: Mastic sealant or foil tape (not regular duct tape, which fails with temperature cycling) can seal small gaps and connections.

Professional duct testing: A technician can pressurize the duct system and measure leakage. Significant duct leaks — common in homes over 15 years old — can reduce system efficiency by 20–30%.

Cost: Duct sealing runs $300–$1,000 for accessible areas; full duct replacement $2,000–$5,000+

Common Causes by System Age

System age often predicts the most likely failure:

0–5 Years (Under Warranty)

Thermostat or control board issues
Improper installation (incorrect refrigerant charge, undersized ductwork)
Manufacturer defects
Filter neglect

What to do: If under 5 years old and not cooling properly, call the installing contractor first — installation defects and startup issues are often covered.

5–10 Years

Low refrigerant from developing leaks
Dirty evaporator or condenser coils from maintenance gaps
Capacitor failures (most common cause of “won’t start” calls)
Contactor wear

Capacitor replacement is one of the most common HVAC repairs. A capacitor helps start the compressor and fan motors. Cost: $150–$400 installed.

10–15 Years

All of the above, plus:

Compressor wear
Blower motor issues
Coil corrosion (especially in coastal environments)

At this age, weigh repair costs against replacement value. A repair that costs more than one-third the cost of replacement is usually not worth it.

15+ Years

All components showing wear
R-22 systems losing refrigerant
Efficiency significantly below modern standards

A 15-year-old system runs at 10–12 SEER. Modern systems are 16–22 SEER — 30–50% more efficient. With major repairs, replacement is almost always the better financial decision.

When to Call a Technician Immediately

Don’t wait or DIY:

Gas smell — leave the home and call your utility
Burning smell from vents — electrical issue, possible fire hazard
Loud banging or grinding — compressor or motor failure risk
Ice buildup won’t clear — refrigerant leak likely
Tripped breaker won’t stay reset — electrical fault in system
System over 15 years old with major failure — get a replacement quote rather than emergency repair

DIY Troubleshooting Checklist Summary

Work through this list before calling:

Thermostat set to COOL and set below current temp?
Air filter checked and replaced if dirty?
Both circuit breakers on and functioning?
All supply vents open?
No ice on refrigerant lines or air handler?
Outdoor unit clear of debris and obstructions?
Condenser fan spinning when system runs?
All accessible duct connections intact?

If all of these check out and the system still isn’t cooling, a technician visit is warranted.

What a Diagnostic Visit Costs

An HVAC diagnostic call runs $75–$150, often waived if you proceed with repairs. Expect the tech to:

Check system pressures with manifold gauges
Verify refrigerant charge
Inspect electrical components (capacitors, contactor, wiring)
Check airflow and temperature differential
Inspect coils (with access)

Get a written diagnosis before authorizing any repairs. A legitimate HVAC company will explain what they found and show you the evidence.

Repair vs. Replace Decision

Use the 5,000 rule as a rough guide: Multiply your system’s age by the repair cost. If the number exceeds $5,000, replacement is typically the better value.

Example: 12-year-old system, $600 compressor repair = $7,200 → lean toward replacement

Modern high-efficiency systems (16+ SEER) can cut cooling costs 20–40%. Federal tax credits (up to $600 for eligible systems), state rebates, and utility incentives may offset 10–30% of replacement cost.

Get quotes from two or three contractors before committing to a major repair on an aging system.

AC Not Cooling: Troubleshooting Checklist and When to Call a Tech