Diagnose Your HVAC Emergency
Eight of the most-searched AC and heating failures, with the likely cause, a quick self-check, and the conditions that mean you should call a licensed technician right now.
(855) 321-3346 — Emergency ServiceAC Running But Not Cooling
If the air handler is on but the air coming out is room-temperature or lukewarm, the cooling cycle is broken somewhere between the thermostat and the compressor. The most common culprits are refrigerant level, a frozen evaporator coil, or a tripped outdoor unit.
Likely causes
- Low refrigerant from a slow leak — system can run but won't reach setpoint
- Frozen evaporator coil from restricted airflow (clogged filter, blocked return)
- Outdoor condenser unit tripped or breaker flipped
- Capacitor failure on the compressor or fan motor
- Dirty condenser coil reducing heat rejection
Self-check before you call
- 1Check the thermostat is set to COOL and 5°F below room temperature
- 2Change the air filter if it has been more than 60 days
- 3Walk to the outdoor unit and confirm the fan is spinning when the system is calling for cool
- 4Reset the AC breaker once — if it trips again, stop and call a technician
Call a technician now if
- Outdoor temperature is above 95°F and indoor temperature is climbing
- Anyone in the home is elderly, very young, or has a heart or respiratory condition
- The unit is making clicking or buzzing sounds without cooling
- Ice is visible on the indoor coil or copper refrigerant lines
AC Blowing Hot Or Warm Air
Hot air from the vents while the system is calling for cool usually means the compressor isn't engaging — either it's tripped on its safety, lost refrigerant, or the reversing valve (on heat pumps) is stuck in heat mode.
Likely causes
- Compressor not running due to capacitor failure or thermal overload trip
- Refrigerant charge fully depleted from a leak
- Heat-pump reversing valve stuck in heating position
- Outdoor disconnect switch turned off (often near the condenser)
- Thermostat wiring fault sending only the fan signal
Self-check before you call
- 1Confirm the outdoor unit is running (listen for the compressor and watch the fan)
- 2Set the thermostat from COOL to OFF for 10 minutes, then back to COOL
- 3Verify the outdoor disconnect (gray box near the condenser) is in the ON position
Call a technician now if
- The outdoor fan is spinning but the compressor never starts
- You smell a chemical or oily odor near the outdoor unit (suggests refrigerant leak)
- The indoor unit is producing condensation but no cool air
AC Won't Turn On At All
Complete failure to start usually points to a power-side problem: thermostat batteries, a tripped breaker, a blown low-voltage fuse on the air handler, or a failed contactor in the outdoor unit. Less commonly, the float switch in the condensate line has cut power because the drain pan is full.
Likely causes
- Dead thermostat batteries (digital thermostats fail silently when batteries die)
- Tripped breaker in the main panel for the AC or air handler circuit
- Blown 3-amp or 5-amp low-voltage fuse on the air-handler control board
- Float switch on the condensate drain line has tripped (full drain pan)
- Failed contactor or transformer in the outdoor unit
Self-check before you call
- 1Replace the thermostat batteries (most accept AA or AAA)
- 2Check the main breaker panel for any tripped breakers, especially labeled HVAC, AC, or air handler
- 3Look for a float switch near the indoor unit's drain line — if tripped, the drain pan is likely full and needs to be cleared
Call a technician now if
- Breaker trips again immediately after reset (indicates a short)
- You hear a buzzing sound from the outdoor unit but nothing starts (failed contactor)
- There is standing water under the indoor unit
AC Leaking Water Inside The House
Water dripping from the indoor unit or pooling near it almost always traces to the condensate drain — either it's clogged, the drain pan is rusted through, or the condensate pump has failed. Left unchecked, this causes ceiling damage and can trip the float switch and shut the AC off entirely.
Likely causes
- Clogged condensate drain line (algae and biofilm buildup is the most common cause)
- Cracked or rusted drain pan under the evaporator coil
- Failed condensate pump (units in basements or attics)
- Frozen evaporator coil that's now thawing rapidly
- Refrigerant leak causing the coil to ice over then drip
Self-check before you call
- 1Turn the AC OFF immediately to prevent further water damage
- 2Locate the PVC drain line outside the home; it should drip steadily when the AC runs
- 3If you have a wet/dry vacuum, you can attach it to the drain line outside and suction out the clog
- 4Check whether the air filter is restricting airflow (frozen coil cause)
Call a technician now if
- Water is dripping through a ceiling below an attic-mounted unit
- The drain pan is visibly rusted or cracked
- Suction-clearing the drain line does not stop the leak
- The leak smells musty (suggests mold in the drain line or pan)
Ice Forming On The AC Or Refrigerant Lines
Visible ice on the copper refrigerant lines or the indoor evaporator coil means the system is either starved of airflow or low on refrigerant. Running a frozen AC damages the compressor — turn it off immediately and let the ice melt before doing anything else.
Likely causes
- Clogged air filter restricting airflow over the evaporator coil
- Blocked or closed return vents reducing airflow
- Low refrigerant charge from a slow leak
- Failed blower motor not moving enough air
- Outdoor temperature too low for the system's design range
Self-check before you call
- 1Turn the thermostat to OFF and the fan to ON — this thaws the coil without further cooling
- 2Replace the air filter while the system is off
- 3Open all supply and return vents in the home
- 4Wait 1-3 hours for the ice to fully melt before restarting
Call a technician now if
- Ice returns within 24 hours of restart
- You see ice on the outdoor unit's refrigerant connections
- There is oily residue near the indoor coil or refrigerant lines (refrigerant leak indicator)
General AC Failure — Where To Start
If the AC simply isn't working and you're not sure why, work through the symptoms above in order: is the unit running but not cooling, blowing warm air, not turning on, or leaking? Each symptom narrows the diagnosis. A few quick checks resolve about a third of reported failures without a service call.
Likely causes
- Most common: dirty filter, dead thermostat batteries, tripped breaker
- Common: capacitor failure on the compressor or fan motor (5-10 year part)
- Seasonal: condensate drain clogged after spring start-up
- Storm-related: lightning surge damage to the control board
- Age-related: refrigerant leak, compressor failure (typical on units over 12 years)
Self-check before you call
- 1Confirm the thermostat is on COOL and set below room temperature
- 2Replace the air filter if you can't remember the last change
- 3Check the breaker panel and the outdoor disconnect switch
- 4Listen at the outdoor unit — fan should spin, compressor should hum
Call a technician now if
- More than one of the symptoms above applies at the same time
- The unit is over 12 years old and this is the second failure in a season
- You see, hear, or smell anything unusual at the outdoor or indoor unit
Furnace Running But Blowing Cold Air
Cold air from the supply vents during a heating cycle means the burner isn't firing, the burner is firing but cutting out (flame sensor or limit switch), or the fan is starting before the heat exchanger warms. On gas furnaces this can also signal a pilot or ignition problem.
Likely causes
- Pilot light is out (older gas furnaces with standing pilots)
- Dirty flame sensor causing the burner to ignite and immediately shut off
- Tripped high-limit switch from a previous overheat
- Closed gas valve or interrupted gas supply
- Thermostat set to FAN ON instead of AUTO (fan runs even when not heating)
Self-check before you call
- 1Verify the thermostat is set to HEAT and the fan setting is AUTO
- 2Confirm the gas valve at the furnace is in the ON position
- 3Look through the inspection window — is the burner firing when the system calls for heat?
- 4If you smell gas, leave the home immediately and call your gas utility, then a technician
Call a technician now if
- You smell gas anywhere in the home
- Carbon monoxide detector is sounding
- Burner ignites then shuts off within 10 seconds (flame sensor failure)
- Outdoor temperature is below freezing and the furnace cannot maintain temperature
Heater Not Working — Gas Furnace, Heat Pump, Or Boiler
Heater symptoms vary by system type. Gas furnaces fail at the ignition, flame sensor, or gas valve. Heat pumps fail at the reversing valve, defrost board, or refrigerant charge. Boilers fail at the circulator pump, expansion tank, or pressure relief. Identify the system type first, then narrow from there.
Likely causes
- Gas furnace: pilot, ignition module, flame sensor, gas valve, or limit switch
- Heat pump: reversing valve stuck in cool, defrost board failure, low refrigerant in cold weather
- Boiler: circulator pump failure, low water pressure, expansion tank waterlogged
- All systems: thermostat failure, tripped breaker, blown low-voltage fuse
- All systems: clogged air filter (furnaces/heat pumps) or air-bound radiator (boilers)
Self-check before you call
- 1Identify the heating system type (gas furnace, electric furnace, heat pump, boiler)
- 2Replace the thermostat batteries and confirm the HEAT setting
- 3Check the main breaker panel for any tripped breakers
- 4Replace the air filter if applicable (furnaces and heat pumps)
Call a technician now if
- Outdoor temperature is below 40°F and the system cannot maintain temperature
- Any gas odor — leave the home and call the gas utility first, then a technician
- Anyone in the home is elderly, very young, or has a health condition affected by cold
- The system makes loud banging, rumbling, or screeching sounds when it tries to start
Not sure which symptom matches?
Describe what you're seeing and a licensed local technician will help diagnose. Service availability subject to technician schedules.
(855) 321-3346