Symptom guides
HVAC symptoms: what that noise, smell, leak or alarm can mean
Each guide collects what official agencies and equipment manufacturers say about one symptom: what people report, what can cause it, what you can check yourself, and where the line is for calling a technician. None of them is a diagnosis.
By the HeatAndAirCost editors · Last updated October 5, 2026 · How we research prices
Emergency first
Smell gas or rotten eggs? Leave on foot and call 911 or your gas utility from outside. CO alarm sounding? Get outside to fresh air and call 911. Smoke or flames? Get out, stay out, call 911. The full official instructions are on the gas, CO alarm and burning smell pages.
Smells and alarms: safety first
These pages open with the official safety instruction. Read that part before anything else.
CO detector going off
A sounding carbon monoxide alarm, a low-battery chirp, and an end-of-life signal mean different things. What CPSC and NFPA say to do for each.
Safety firstFurnace smells like gas
The federal pipeline-safety instruction for a suspected gas leak, who shuts the gas off, and who repairs the line inside your home.
Safety firstFurnace smells like rotten eggs
Why gas companies make natural gas smell like rotten eggs or skunk, and why even a faint whiff gets the full leak response.
Safety firstFurnace smells like burning
First-run dust smell versus burning plastic or electrical odor: how Trane, Carrier and NFPA separate the two, and what to do for each.
AC not cooling right
Warm air, water, ice, or no power at all.
AC blowing warm air
Is the outdoor unit running at all? The two-minute check that splits a warm-air problem into its likely halves.
SymptomWater leaking from AC
Drain pan, drain line, frozen coil or refrigerant: the four sources manufacturers name, and how to limit water damage today.
SymptomAC not turning on
Thermostat, power, airflow, drain switch: the order manufacturers check a dead AC, and the breaker rule that trips people up.
SymptomIce on the AC line
Frost on the copper refrigerant line usually points back to a frozen coil. How long a safe thaw takes and what not to do.
Noises from the AC
Buzzing, hissing, clicking and squealing, and where each sound is coming from.
AC buzzing noise
A buzz or hum from the outdoor unit is most often tied to the capacitor, contactor or fan motor. Where the sound comes from matters.
Safety firstAC hissing noise
Hissing at the refrigerant lines, at the outdoor unit, or near a gas furnace each point somewhere different. One of them is a gas call.
SymptomAC clicking noise
One click at start and stop versus clicking that never settles. What to note before you call, per Trane's troubleshooting guide.
SymptomAC squealing noise
High-pitched noise from the outdoor unit or the air handler, the filter test you can do safely, and when to switch it off.
Furnace, heat pump and thermostat
Cycling, steam, and a thermostat that went dark.
Furnace short cycling
A furnace that keeps shutting off before the house warms up. Thermostat placement, airflow, a blocked flue, and sizing.
SymptomThermostat blank screen
A dark thermostat means no power reaching it. Batteries, the furnace door switch, a tripped drain-pan switch, or a breaker.
SymptomHeat pump steaming
Steam from the outdoor unit in cold, damp weather is how manufacturers describe a defrost cycle. When ice that will not clear changes that.
Comfort and cost
Hot and cold rooms, and summer bills that jumped.
Uneven room temperatures
One room hotter or colder than the rest. ENERGY STAR's duct-leak numbers and the register, damper and thermostat checks.
SymptomHigh electric bill from AC
How much of a home's electricity goes to air conditioning (EIA), and the federal figures for filters, ducts and thermostats.
Step-by-step fixes
Our earlier troubleshooting guides walk through the free checks for the most common no-heat and no-cool calls. Every guide, symptom and step-by-step alike, is on the HVAC troubleshooting hub.
How these guides are built
Safety instructions are quoted from the agency that issues them (CPSC, PHMSA, NFPA). Causes are limited to what a cited federal source (DOE, ENERGY STAR, EPA, EIA) or a manufacturer’s own troubleshooting page says, and each guide lists its sources. Some guides include Amazon affiliate links for screening or upkeep items, labeled where they appear, and a partner quote module labeled “Advertising partner.” See our disclosure and methodology.