Most HVAC systems stop cooling or heating properly for one of five reasons: a wrong thermostat setting, a clogged air filter, a tripped breaker, blocked airflow, or a failing part such as a capacitor, igniter or refrigerant line. The first four you can often fix yourself in minutes. The fifth needs a trained technician.

Whether your living room feels like a sauna in July or a fridge in January, the steps below help you find the cause fast, fix what is safe to fix, and know exactly when to call for help.

Check These 5 Things First (10 Minutes, No Tools)

Many "broken" systems are not broken at all. Work through this list before you book a service call.

1. Thermostat settings. Make sure it is set to COOL or HEAT, not just FAN, and that the set temperature is a few degrees past the room temperature. Replace the batteries if the screen is dim or blank.
2. Air filter. Pull it out and hold it to the light. If you cannot see light through it, replace it. The U.S. Department of Energy says routinely replacing a dirty filter can cut an air conditioner's energy use by 5–15%.
3. Circuit breaker and switches. Check the breaker panel for a tripped HVAC breaker. Also check the service switch near the indoor unit, which looks like a light switch and is easy to flip off by mistake.
4. Vents and registers. Open every supply vent and move furniture or rugs off return grilles. Closing vents in unused rooms often makes the whole system work worse, not better.
5. Outdoor unit. Clear leaves, grass and debris for at least 2 feet around it. A smothered condenser cannot release heat.

Why Your AC Is Running but Not Cooling

If the quick checks did not solve it, one of these is the likely cause.

• Low or leaking refrigerant. Refrigerant is not "used up" like fuel. If it is low, there is a leak. Signs include hissing sounds, warm air from vents and ice on the copper lines. Only a certified technician can find the leak and recharge the system.
• Frozen evaporator coil. Restricted airflow or low refrigerant can make the indoor coil ice over. Turn the system to FAN only for a few hours to thaw it, then fix the cause.
• Dirty condenser or evaporator coils. A layer of dust acts like a blanket and blocks heat transfer. The DOE recommends checking the evaporator coil every year.
• Failed capacitor or contactor. If the outdoor fan is not spinning, or the unit hums but will not start, a small electrical part has often failed.
• Leaky or crushed ducts. Cool air that escapes into an attic or crawl space never reaches your rooms.

Why Your Furnace or Heat Pump Is Not Heating

Heating problems follow a similar pattern, with a few parts unique to furnaces and heat pumps.

• Dirty flame sensor. A thin coat of carbon makes the furnace light, run for a few seconds, then shut off. Cleaning it is a quick, inexpensive fix for a technician.
• Failed igniter or pilot. If you hear the blower but the burners never light, the igniter has often cracked or burned out.
• Blocked condensate drain. High-efficiency furnaces produce water. A clogged drain line trips a safety switch and shuts the furnace down.
• Heat pump stuck in defrost or the wrong mode. A faulty reversing valve can leave a heat pump blowing cool air in winter.
• Gas supply issue. If other gas appliances also fail, the problem may be the supply. If you ever smell gas, leave the house first and call your gas company from outside.

Symptom Finder: What It Means and Who Should Fix It

Match what you are seeing to the most likely cause.

When to Call an HVAC Repair Company

Call a professional when the quick checks do not help, or straight away if you notice any of these:

• Ice on the refrigerant lines or indoor coil that returns after thawing
• Burning, electrical or rotten-egg smells
• Loud grinding, banging or screeching noises
• A breaker that trips again after you reset it
• Water pooling around the furnace or air handler
• A carbon monoxide alarm, which means leaving the home first

Refrigerant, gas lines and high-voltage parts are not safe DIY territory. A licensed HVAC repair company can test refrigerant pressure, check electrical components and inspect the heat exchanger in a single visit, often finding small faults before they turn into a full breakdown.

Before the technician arrives, write down what you noticed and when it started. Clear details shorten the visit and usually lower the bill.

How to Stop It Happening Again

Most cooling and heating failures start as small problems nobody noticed. A simple routine prevents the majority of them.

• Check the air filter every month and replace it at least every 1–3 months.
• Book a professional tune-up twice a year: the AC in spring, the heating in autumn.
• Keep 2 feet of clear space around the outdoor unit all year.
• Pour a cup of diluted vinegar down the condensate drain each season to stop clogs.
• Use a programmable thermostat. The DOE estimates it can save up to 10% a year on heating and cooling.
• Test carbon monoxide detectors when the clocks change.

Frequently Asked Questions

Why is my AC running but the house is not cooling down?

The most common causes are a clogged filter, a dirty outdoor coil, low refrigerant or a frozen indoor coil. Check the filter and outdoor unit first. If there is ice or warm air, call a technician.

Why does my furnace turn on and then shut off right away?

This is called short cycling. A dirty flame sensor is the usual cause, followed by a clogged filter or an oversized system.

Can I add refrigerant to my AC myself?

No. Low refrigerant means a leak, and handling refrigerant requires EPA certification in the U.S. Topping it up without fixing the leak only delays the next breakdown.

How often should my HVAC system be serviced?

Twice a year is ideal: once before summer for cooling and once before winter for heating.

Is it normal for one room to be hotter or colder than the rest?

A small difference is normal. A large one usually points to duct leaks, blocked vents or poor insulation in that room.