Problems

HVAC problems and troubleshooting

23 symptom guides. Safe checks first, and a clear line where the job needs a licensed technician.

Updated October 2026 · Data as of Sources checked October 26, 2026

Written by HYRE HVAC Research Desk Primary-source research, data analysis and fact checking

The short version

Start HVAC troubleshooting with the safe checks: thermostat, breaker, filter and drain. If you smell gas or a CO alarm sounds, leave the house and call from outside. Each guide below covers one symptom, says what you can check yourself, and where to stop.

Safety first: when to stop troubleshooting

You smell gas

Leave the house, then call your gas utility or 911 from outside. Do not switch anything on or off.

A carbon monoxide alarm sounds

Get everyone to fresh air, then call 911. CPSC advises CO alarms on every level and outside sleeping areas.

A breaker trips again after one reset

Leave it off and call a licensed contractor.

Refrigerant and line-voltage work

Refrigerant handling needs EPA Section 608 certification. Leave panels and wiring to a licensed technician.

Air conditioner and heat pump problems

AC unit freezing up An AC unit freezing up means the indoor coil dropped below freezing. Troubleshoot AC unit not turning on An AC unit not turning on is most often a thermostat problem, a tripped breaker, an outdoor disconnect switched off, the indoor unit’s service switch turned off, or a float switch that shut the system down because the drain pan is full. Troubleshoot Air conditioner fan not spinning An air conditioner fan not spinning, with the outdoor unit humming, most often has a failed run capacitor, a failed fan motor, a stuck contactor or debris jamming the blade. Troubleshoot Air conditioner leaking water An air conditioner leaking water usually has a clogged condensate drain line, a full or cracked drain pan, a frozen evaporator coil that is thawing, a failed condensate pump, or a disconnected drain. Troubleshoot Air conditioner not working When an air conditioner is not working, sort it by symptom. Troubleshoot Air conditioner short cycling Air conditioner short cycling means the system runs only a few minutes before shutting off, then restarts soon after. Troubleshoot How an AC condenser fan motor is replaced Replacing an air conditioner condenser fan motor means isolating power, discharging and testing the capacitor, removing the top grille and fan assembly, matching a new motor by horsepower, voltage, speed and rotation, wiring it correctly and resetting the blade. Troubleshoot How to clean a window AC filter To clean a window AC filter, unplug the unit, open or remove the front grille, slide out the mesh filter, vacuum off loose dust, rinse it with lukewarm water and mild soap, let it dry completely, and slide it back in. Troubleshoot How to clean air conditioner mold To clean air conditioner mold on a window unit, unplug it, remove and wash the filter, and wipe reachable surfaces with mild detergent and water, then dry them fully. Troubleshoot How to fix an AC unit To fix an AC unit yourself, stick to the thermostat, the filter, the condensate drain’s float switch, the outdoor unit’s surroundings and a single breaker reset. Troubleshoot How to reset an AC unit To reset an AC unit, turn the thermostat to off, switch off the breaker for the outdoor unit (and the indoor unit if separate), wait a few minutes, switch the breakers back on, then set the thermostat to cool. Troubleshoot How to turn on an air conditioner To turn on an air conditioner, set the thermostat mode to cool, set the temperature a few degrees below the room, and leave the fan on auto. Troubleshoot Refrigerant leak A refrigerant leak shows up as weaker cooling, ice on the refrigerant lines or indoor coil, a hissing or bubbling sound, or longer run times. Troubleshoot Why is my heat pump not cooling? A heat pump not cooling is most often a thermostat set to heat or auto, a clogged filter, a blocked or dirty outdoor coil, or a tripped breaker. Troubleshoot

Furnace, heating and thermostat problems

Furnace blower not working A furnace blower not working is usually a thermostat fan setting, a door safety switch that is open, a tripped breaker, a failed blower capacitor, a burned-out motor or a control board fault. Troubleshoot Furnace keeps tripping the breaker A furnace that keeps tripping the breaker has an electrical fault: often a failing blower motor or capacitor, a shorted wire, a failing inducer motor, a fault in an electric furnace’s heating elements, or a circuit that is overloaded or has a worn breaker. Troubleshoot Furnace making a buzzing noise A furnace making a buzzing noise usually has an electrical cause: a failing blower capacitor, a humming transformer, a chattering relay or contactor, a motor struggling to start, or a loose panel vibrating. Troubleshoot Furnace not blowing air A furnace not blowing air usually has a thermostat setting issue, a clogged filter, a furnace door not closed on its safety switch, a tripped breaker or service switch, or a failed blower motor, capacitor or control board. Troubleshoot How to change a thermostat To change a thermostat, first confirm it is a low-voltage (24-volt) model, switch off power to the furnace or air handler at the breaker, photograph and label the wires, then move each wire to the matching terminal on the new thermostat. Troubleshoot How to fix a heat pump To fix a heat pump, an owner can safely check the thermostat, change the filter, clear the outdoor unit and reset a tripped breaker once. Troubleshoot Signs of a bad AC thermostat Signs of a bad AC thermostat include a blank or flickering screen after new batteries, a temperature reading that does not match a separate thermometer, an AC that will not start or will not stop, short cycling, and settings that do not hold. Troubleshoot Thermostat blinking "cool on" A thermostat blinking "cool on" usually means the compressor delay is active. Troubleshoot Thermostat not working A thermostat not working is most often dead batteries, a tripped breaker or service switch on the furnace or air handler, a tripped float switch, a blown low-voltage fuse on the control board, or a loose wire. Troubleshoot

When a problem needs a professional

Each page lists the safe checks a homeowner can make, then stops where the work needs a licensed or certified technician.

If the fix is a repair, the matching service guide explains what a good visit includes. If the system is old, the repair or replace calculator helps you decide. For what drives the price, see HVAC cost guides.

Questions

What should I check first when my HVAC stops working?

The thermostat setting and batteries, the breaker, the furnace switch, the filter and the condensate drain.

When should I call a technician?

When a safe check does not fix it, a breaker trips twice, ice keeps returning, or anything involves gas, refrigerant or wiring.

Is a dirty filter really a common cause?

Yes. Low airflow can freeze an AC coil and trip a furnace limit switch.

What if I smell gas?

Leave the house, then call your gas utility or 911 from outside.

Written and audited by

HYRE HVAC Research Desk

Primary-source research, data analysis and fact checking

We are a research desk, not a sales floor. We read the federal microdata file, the statute or the manufacturer data sheet ourselves, and we publish the figure with the document it came from and the date we retrieved it.

Where a number cannot be traced to a primary source, we publish the shorter page and say what we could not verify.

The counts below are generated from the published pages themselves, last counted October 26, 2026, and they are what we have actually published rather than what we intend to.

18: studies published
15: federal sources read and cited
15: studies published with their full dataset as CSV
51: jurisdictions reproduced against EIA’s own tables

How this desk works

Primary sources only. Federal data comes from the agency that collects it, in the file that agency publishes. We do not cite an article that cites a source; we download the source and compute the figure ourselves.
We validate against the agency before we publish. First, we use each federal microdata file to reproduce the agency’s own published tables. Our cooling research reproduces EIA’s state estimates and standard errors for all 51 jurisdictions. That check caught a variance formula that was off by a factor of four.
Every estimate carries its uncertainty. These are survey figures, not counts. Standard errors are computed from the replicate weights the federal file supplies and printed beside the estimate. An estimate too imprecise to publish is reported as such rather than printed.
Nothing is typed by hand. Prose, tables and charts all read from one dataset built by script, so a number in a sentence and the same number in the table below it cannot disagree.
We publish the data, not just the conclusion. 15 of our 18 studies offer the full computed table as a CSV download on the page, so you can check the analysis or disagree with it. A study without a row-level dataset gets no download link and claims none in its structured data.
We correct in public. Where we have published a figure wrongly we fix the figure, rewrite any analysis that rested on it rather than patching the number underneath it, and leave a dated correction note on the page.
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Data as of Sources checked October 26, 2026. Authorship on this site is organizational: the analysis belongs to the desk rather than to a named individual, and we do not publish credentials we do not hold. Our editorial policy sets out how we source, date and correct what we publish.

Sources & retrieval dates

US CPSC: Carbon Monoxide Information Center , CO alarm placement and what to do if you suspect CO poisoning. Retrieved October 26, 2026.

EPA: Section 608 Technician Certification Requirements , Who may work on refrigerant circuits. Retrieved October 26, 2026.

ENERGY STAR: Maintenance Checklist (heating and cooling) , Monthly filter checks and seasonal check-up items. Retrieved October 26, 2026.

HyreHVAC is a publisher. We do not install, repair or inspect HVAC equipment and hold no contractor licence. Nothing here replaces an on-site diagnosis by a licensed contractor.