Troubleshooting
Diagnose your HVAC problem before you call a tech
Most HVAC service calls start with a homeowner Googling a symptom. Half of those calls are problems the homeowner could have fixed in 10 minutes with a screwdriver and a replacement filter. The other half need a licensed tech, but the diagnosis still matters because it tells you whether the bill will be $150 or $1,500.
Each guide below walks through the diagnostic sequence a real service technician runs on a job, in the order they run it. We list the actual part costs, which steps you can do safely on your own, and which ones need a tech with gauges, a clamp meter, and EPA certification. Reviewed by licensed HVAC technicians who do this work every day.
Updated May 2026
Start with your symptom
Each guide is organized around what you're actually seeing or feeling, not the underlying mechanical cause. If you're not sure which symptom matches, start with the closest one and the diagnostic tree will point you at the right next page.
Diagnostic
Portable AC not cooling
A portable that runs but will not cool is usually a setup or sizing problem, not a breakdown. The hand-at-the-vent test, the fixes in order from filter to window-kit seal, why single-hose units fight themselves, and when the box BTU lied about the size.
Diagnostic
AC not turning on
A dead AC is usually a free fix. The thermostat, breaker, power switches, and drain safety switch checks that solve most no-cool calls, then what each repair costs.
Diagnostic
AC short cycling
Why the AC keeps turning on and off, the free checks to try first, the pattern that tells you the cause, and the deeper culprits from a frozen coil to an oversized unit.
Diagnostic
House humid with the AC running
The house is cool but stays sticky. Why the AC is not dehumidifying, the free fan setting that fixes most cases, what to check next, and when an oversized unit or a dehumidifier is the real answer.
Diagnostic
AC smells bad
What a musty, rotten egg, burning, chemical, dirty sock, or vinegar smell from the AC means, which two are safety emergencies to act on now, which you can fix yourself, and which need a tech.
Diagnostic
AC blowing warm air
The diagnostic order a service tech actually runs, from the thermostat to the compressor, with real part costs and which fixes you can do yourself.
Diagnostic
AC running but not cooling
The air is cold but the house never gets there. The capacity and airflow causes, from a dirty condenser to leaky ducts to an undersized unit, and what each fix costs.
Diagnostic
AC making a loud noise
What a buzzing, rattling, banging, screeching, humming, or hissing AC means, sorted by sound, which ones are a safe DIY check, which mean shut it off now, and what each fix costs.
Diagnostic
AC freezing up
Eight causes of a frozen evaporator coil, the ice-location decision tree that tells which one you have, and the gauge readings that separate airflow from refrigerant.
Diagnostic
Furnace not turning on
A dead furnace is usually a free fix. The thermostat, breaker, power switch, access panel, and condensate float checks that solve most no-heat calls, then what repairs cost.
Diagnostic
Furnace blowing cold air
Gas furnace, electric, or heat pump diagnostics in one walk-through, with the LED blink codes, flame sensor readings, and defrost cycle timing a tech actually checks.
Diagnostic
Furnace short cycling
Why the furnace fires up then shuts off every few minutes, the free filter fix to try first, the flame sensor and high-limit switch causes, the oversized-furnace case, and the cracked heat exchanger and carbon monoxide warning that means stop.
Diagnostic
AC leaking water
Where the water shows up tells you what is wrong. Eight causes mapped to four leak locations, plus the trade method for clearing the drain that most homeowner videos get backwards.
Diagnostic
Heat pump not heating
Two branches at the outdoor unit decide the whole diagnostic: running but blowing cold, or not running at all. Heating-mode gauge readings, reversing-valve test, and the $40 fix that techs miss.
Diagnostic
AC tripping the breaker
Two free checks before paying a service call, the eight real causes ranked by how often techs find them, and the one cause almost every online guide misses: the breaker itself.
Diagnostic
Thermostat not working
Blank screen, will not turn on the AC, or the temperature reading is wrong. Seven DIY checks under $10 that fix most calls, plus what the smart thermostat C-wire problem really is.
Diagnostic
Oversized HVAC system signs
Four tests to confirm if your AC is too big, the 10 symptoms of oversizing, and what the fixes cost from a $15 hygrometer to a full system replacement.
Before you call a tech, check these three things
About half the HVAC service calls a tech runs in a typical week turn out to be one of three issues a homeowner could have caught in 10 minutes. None of these are tricks. They are just the things technicians wish you had checked before booking the appointment.
1. The thermostat batteries and the mode setting
Battery-powered thermostats die without warning. The screen still lights up sometimes because the LCD has its own residual charge, but the relay that triggers the heating or cooling call has lost power. Swap the batteries (almost always two AAs behind the faceplate), confirm the mode is set to Cool or Heat (not Off, not Fan), and confirm the setpoint is below the room temperature in summer or above it in winter. This single check resolves about 10 percent of "my AC is broken" calls.
2. The air filter
A clogged air filter is the single most common cause of every comfort complaint: warm air from a cooling system, cold rooms in winter, frozen evaporator coils, short cycling, high electric bills, and even compressor failures over time. Filters should be replaced every 1 to 3 months depending on type and how dirty your home runs. A black, matted filter pulled from the return duct will solve about 25 percent of complaints before a tech ever needs to look at the equipment.
3. The breaker on the outdoor unit
Most homes have a dedicated 30 to 60 amp breaker for the outdoor AC condenser or heat pump, often labeled "AC" or "HP" in the panel. Power surges, lightning strikes, or a weak capacitor can trip this breaker. Reset it once. If it trips again immediately or within an hour, stop and call a tech. That repeated trip is the equipment telling you a component has failed, and continuing to reset it can damage the compressor or start a fire.
When the troubleshooting points at replacement
Some failures are routine repairs (capacitor, contactor, refrigerant top-up). Others are signals that the equipment is at the end of its useful life and the repair money is better spent on a new system. Once a diagnostic guide points you at a repair over $1,500 on a system more than 10 years old, the math usually flips toward replacement. Run the numbers in the replace vs repair calculator before you authorize the work, and read the central AC vs heat pump comparison if you're about to buy a new system anyway.