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HVAC Troubleshooting Assistant

Answer a few questions to see the safe checks worth doing before you call someone, what a technician will measure, and what to do if something is dangerous.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Thermostat is in the right mode, set past the room temperature, with fresh batteries (if it uses them)?
Breakers and the service switch near the equipment are on?

Look only — do not open electrical panels on the equipment.

Air filter is clean (checked in the last month)?
Supply registers and return grilles are open and not blocked?
Outdoor unit is running and clear of leaves and debris?

Years. Used only for the Repair or Replace hand-off.

Leave at 0 if you do not have a quote yet.

Result

5 checks to do first

Checks to do yourself
5

Start with 5 simple checks, then call a technician if the problem remains.

  • Set the thermostat to the right mode (cool or heat), set it a few degrees past the room temperature, and replace its batteries if it uses them.
  • Check that the breakers and the service switch near the equipment are on. A tripped breaker points to an electrical fault: do not keep resetting it — call a licensed technician.
  • Replace or clean the air filter. A clogged filter restricts airflow, lowers capacity and can make the coil ice up or the furnace overheat and cycle.
  • Open supply registers and clear furniture, rugs and curtains away from registers and return grilles.
  • Clear leaves and debris from around the outdoor unit and keep plants trimmed back. If it is not running while the thermostat calls for cooling or heating, call a technician.
  • Low refrigerant means a leak: only an EPA Section 608 certified technician may repair it and recharge the system.
  • If the first checks do not fix it, a technician will typically measure temperature split across the coil, airflow (static pressure), and refrigerant superheat and subcooling — or, for a furnace, temperature rise and gas input.
Show the working
  1. Thermostat
    not checked yet
  2. Power
    not checked yet
  3. Filter
    not checked yet
  4. Vents
    not checked yet
  5. Outdoor unit
    not checked yet

Safety comes first

Some situations are not troubleshooting problems. If you smell gas, get everyone out and call the gas company or 911 from a safe place, without switching anything on or off. If a carbon monoxide alarm sounds or people feel dizzy or sick, get outside to fresh air and call 911. Burning smells, smoke, sparks or water near electrical parts mean turning the system off at the thermostat and calling a professional.

The checks anyone can do

Many service calls turn out to be simple: a thermostat in the wrong mode or with flat batteries, a tripped breaker or switched-off service switch, a clogged filter, closed or blocked registers, debris around the outdoor unit, or a blocked condensate drain that has tripped a safety switch. These checks need no tools and no access inside the equipment.

The U.S. Department of Energy lists dirty filters, thermostat settings and blocked drains among the most common air conditioner problems, and ENERGY STAR recommends checking the filter every month.

What this tool will not do

It gives no instructions for work on gas valves, burners, refrigerant circuits or line-voltage electrical parts, and it does not interpret fault codes. Refrigerant work requires EPA Section 608 certification, and gas and electrical repairs need a licensed professional. If the first checks do not solve the problem, it lists the measurements a technician will typically take, and links to our calculators for them.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Is any of these happening now?
None of these
System
Central air conditioner or heat pump (cooling)
What is happening?
It runs but does not cool or heat enough
Thermostat is in the right mode, set past the room temperature, with fresh batteries (if it uses them)?
Not checked yet
Breakers and the service switch near the equipment are on?
Not checked yet
Air filter is clean (checked in the last month)?
Not checked yet
Supply registers and return grilles are open and not blocked?
Not checked yet
Outdoor unit is running and clear of leaves and debris?
Not checked yet
Age of the system
12
Repair quote, if you have one
0 $

Result

Checks to do yourself
5

Start with 5 simple checks, then call a technician if the problem remains.

Assumptions

  • Only checks that need no tools and no access inside the equipment: thermostat settings, breakers and switches, filter, registers, the outdoor unit’s surroundings and a visible condensate drain.
  • Safety situations (gas smell, carbon monoxide, burning smell, water near electrical parts) stop the assistant and show what to do, from government safety guidance.

Limitations

  • Preliminary guidance, not a diagnosis. It gives no instructions for gas, refrigerant-circuit or line-voltage work, and does not interpret manufacturer fault codes.
  • Anything involving gas, refrigerant, electrical components or the warranty needs a licensed or certified professional.

Frequently asked questions

My AC runs but the air is not cold. What should I check?

The thermostat mode and setting, the filter, that registers are open, and that the outdoor unit is running and clear of debris. If all are fine, a technician will measure the temperature split across the coil, airflow and the refrigerant charge.

Why is there ice on my indoor coil?

Usually too little airflow — a dirty filter or blocked returns — or a refrigerant problem. Switch cooling off and let it melt completely. If it ices again after the filter is replaced, call a technician.

Should I repair or replace my system?

Once you have a repair quote, the Repair or Replace Calculator compares the cost of repairing now with replacing, over the same period. The assistant passes your system’s age and quote to it.

Sources

  1. Maintenance Checklist (heating and cooling) — ENERGY STAR (U.S. EPA). Inspect, clean or change air filters once a month; a contractor checks thermostat settings, the condensate drain (a clogged drain can cause water damage), electrical connections, refrigerant level and airflow.
  2. Energy Saver 101: Home Cooling (infographic) — U.S. Department of Energy. Common air conditioner problems: low or leaking refrigerant (call a trained technician), thermostat settings and batteries, clogged drains, dirty filters, electric control failure and duct problems; maintenance: replace or clean filters, check the coil yearly, pass a stiff wire through drain channels, clear debris from the outdoor unit, hire a certified professional for more than basic maintenance.
  3. Consumer Guide to Home Heating and Cooling (2021) — U.S. Department of Energy, Energy Saver. Hire a professional HVAC technician to inspect systems: air conditioners in the spring and furnaces in the fall; clean or replace filters monthly during seasons of peak use.
  4. Carbon Monoxide Information Center — U.S. Consumer Product Safety Commission. If you suspect carbon monoxide poisoning, get outside to fresh air immediately and then call 911; symptoms include headache, dizziness, weakness, nausea, vomiting, sleepiness and confusion.
  5. What To Do If You Smell Natural Gas — Connecticut Public Utilities Regulatory Authority. If the smell is strong, get everyone out and call the gas company from elsewhere; avoid sparks (light switches, matches, smoking); do not return until the gas company says it is safe; never turn the gas back on yourself.
  6. Section 608 Technician Certification — U.S. Environmental Protection Agency. Technicians who maintain, service, repair or dispose of equipment that could release ozone-depleting refrigerants into the atmosphere must be certified.

Spotted an error? Report a problem with this calculator — see our corrections policy.