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Guide Updated October 3, 2026

HVAC Not Heating or Cooling? What to Check

What to check when the heating or cooling will not run: thermostat, power, filter and vents, and the signs that mean a technician is needed.

Key points

  • Work the six quick checks first: thermostat, breaker, filter, vents, outdoor unit, one restart.
  • That fixes most airflow causes by itself.
  • Everything that involves refrigerant, gas or electricity belongs to a licensed technician.

When the heating or the cooling stops, the cause is often something simple and safe to check: a thermostat on the wrong setting, a tripped breaker, a blocked filter or a covered vent. Work through the short list below in order. If everything checks out and the problem stays, the next step is a technician.

Safe sequence for checking thermostat settings, power, filter, registers and the outdoor unit before stopping for professional service
What to notice: Check accessible controls and airflow in order. Stop before opening equipment or touching refrigerant and electrical components. Conceptual illustration; exact equipment and layout vary.

The common causes and which of them are safe to check

Airflow problems (safe to check yourself)

A blocked filter, a closed vent or a covered return means too little air moves through the system. In winter the furnace overheats and shuts its burners off. In summer the indoor coil freezes. Fixing the airflow often fixes both.

Frozen indoor coil

Ice on the indoor coil means restricted airflow or a low refrigerant charge. Turn the cooling off at the thermostat, switch the fan to On, and let the fan run until the ice is gone. Replace the filter and open the vents. If the ice returns, the cause is elsewhere and a technician needs to see it.

Frozen or blocked outdoor unit

The outdoor unit releases the heat the system removes from the house. If it is buried in leaves or ice, or if its fan does not spin, it cannot do its job.

Refrigerant and electrical faults (technician’s job)

Refrigerant does not get used up: a low charge means a leak, and finding and fixing a leak takes training and a license. The same goes for electrical parts inside the equipment. These are the jobs where the safe answer is to stop and call.

Call for heating repair or air conditioning repair when:

What the technician will look at

A technician measures what the homeowner cannot: the refrigerant charge, the electrical parts, the safety controls, the fans and motors, the burners and the heat exchanger. The goal is the cause, not just the symptom, so the same small check list above becomes a proper diagnosis on a service visit.

Repair or replace

Most causes are repairs. Replacement becomes the real question when the system is old and a main part fails — our guide on what HVAC repair costs and what drives the price explains the main points.

Sources

Need help with this?

All services

Our services page lists every service and how to book it. We assess the system, explain what we found and provide a written quote for approval before work begins.

Before you call

Safe checks, and what needs a professional

When to call a technician

  • the checks above are all fine and the system still will not run;
  • the breaker trips again, or the outdoor unit's fan does not spin;
  • ice forms on the indoor coil or the copper lines after a thaw;
  • the system runs constantly but the house does not reach the set temperature;
  • you hear grinding, squealing or banging, or smell burning;
  • water appears around the indoor unit.

Safety. You smell gas: Do not use switches or phones inside. Go outside, then call your gas company or 911. The carbon monoxide alarm sounds: Get everyone into fresh air, then call 911.

Quick checks, in order

  • The thermostat. It should be set to Heat or Cool as needed, with the fan on Auto and the target temperature above or below the room temperature. Replace the batteries if the screen is dim or blank.
  • The breaker. Find the electrical panel and look for a breaker that has tripped to the middle or off position. Reset it once. A breaker that trips again is reporting a real fault.
  • The filter. Pull it out and hold it up to a light. If you cannot see light through it, replace it with a new one of the same size. Our filter guide explains sizes and ratings.
  • The vents. Walk the house. Open the supply vents and make sure furniture, rugs or curtains do not cover a vent or a return grille.
  • The outdoor unit. Clear leaves, grass, snow and ice from around it, and make sure the small switch or disconnect box beside it is in the on position.
  • Restart once. Switch the system off at the thermostat, wait a few minutes, then set it again. One restart is a reasonable test. Repeated restarts of a misbehaving system can make the problem worse.

What you should not do

  • Do not add refrigerant or attempt any work on the refrigerant lines.
  • Do not open the equipment cabinets or work on electrical parts.
  • Do not reset a tripped breaker more than once.
  • Do not keep restarting the system after it shuts itself off.

Common questions about an HVAC system that will not heat or cool

How often should I change the air filter?
Look at it monthly and change it when it looks dirty. One-inch filters often need changing more often than thick filters, and homes with pets or dust need more frequent checks.
How can I keep the system working well?
Change the filter when it is dirty, keep the vents and the outdoor unit clear, book one maintenance visit a year, and note what the system does differently so you can describe it when you call.
The system runs, but some rooms stay uneven. Is that a duct problem?
It can be. Leaky or badly laid out ducts leave some rooms behind. See duct repair for the sealing service and the signs of a duct problem.

Heating and cooling service · Hillside IL

Heating and cooling service in Hillside IL

SquareHVAC repairs, replaces and maintains heating and cooling systems for homes and businesses in Hillside IL and the surrounding communities.

We inspect the equipment, explain what we find and provide a written quote with parts and labor listed separately. You decide if and when the work happens. Serving homeowners and businesses since 2009.

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  • Serving since 2009