Troubleshooting
HVAC Troubleshooting Guide
Quick answer
Nearly all HVAC faults present as one of seven symptoms: no power, no heating, no cooling, weak airflow, unusual noise, water where it should not be, or odors. Check thermostat, breakers, filter, and condensate drain first; those four items resolve about a third of all service calls.
The universal first four checks
Whatever the symptom, start here. These take ten minutes, cost nothing, and are the first things a paid technician would check anyway.
Thermostat
Correct mode, fan on Auto, setpoint beyond current room temperature, fresh batteries, display alive.
Power
Both system breakers, the furnace service switch, and the outdoor disconnect. Reset a trip once only.
Filter
If light does not pass through it, replace it. Restricted filters cause weak airflow, freezing, overheating, and short cycling.
Condensate
Standing water in the pan or a triggered float switch shuts systems down by design. Clear the drain line, often with a wet dry vacuum at the outdoor termination.
Symptom directory
Once the basics pass, use the symptom to route to the detailed guide or the right professional conversation.
| Symptom | Most likely causes | Where to go next |
|---|---|---|
| AC runs, air not cold | Refrigerant, dirty coils, capacitor | AC blowing warm air guide |
| No heat from furnace | Ignitor, flame sensor, gas supply | Furnace repair signs guide |
| Heat pump weak in cold | Often normal, else defrost or aux heat | Heat pump not heating guide |
| Weak airflow everywhere | Filter, blower, duct leaks or crushing | Maintenance checklist |
| Ice on refrigerant lines | Airflow restriction or low charge | Frozen coil guide |
| Water around indoor unit | Clogged condensate drain, cracked pan | Call if drain clearing fails |
| Burning dust smell at first heat | Normal for first fall firing | Self resolves in hours |
| Electrical or fishy odor | Overheating wiring or motor | Shut down, call same day |
| Rotten egg smell | Gas leak | Leave home, call gas utility |
Know when to stop
Homeowner troubleshooting ends where refrigerant circuits, gas combustion, and high voltage begin, both for legal reasons and for safety. Stop and call a professional when a breaker trips twice, when any gas or strong electrical odor appears, when diagnosis requires opening sealed panels, or when the fix involves refrigerant. Document what you checked and what you observed; it shortens the technician's diagnostic time and often lowers the bill.
Frequently asked questions
What should I check before calling an HVAC company?
Thermostat settings and batteries, both breakers plus the furnace switch and outdoor disconnect, the air filter, and the condensate drain and float switch. Note the symptom precisely, what runs, what does not, and any sounds, and you may fix it yourself or at least cut the diagnostic bill.
Why did my whole HVAC system suddenly stop?
Total shutdowns are usually electrical or safety switches: a tripped breaker, a failed transformer or blown board fuse, a float switch in a full condensate pan, or a thermostat power loss. Genuine simultaneous mechanical failure of both heating and cooling is rare.
How often do HVAC systems need professional service?
Twice a year, a cooling tune up in spring and a heating inspection in fall, is the industry standard. Annual service at minimum. It keeps warranties valid, catches wear parts before failure, and maintains rated efficiency, typically paying for itself in energy alone.
Related guides
- AC Not Cooling? Step by Step FixesA systematic homeowner checklist for an air conditioner that will not cool, from thermostat to condenser, with realistic repair costs.
- 10 Signs Your Furnace Needs RepairThe warning signs of a failing furnace, from short cycling to yellow burner flames, ranked by urgency including the signs that mean shut it down now.
- Thermostat Not Working? Fixes That Take 5 MinutesHow to revive a dead or unresponsive thermostat, from batteries and breakers to the float switch trick, and when the problem is not the thermostat at all.
- HVAC Maintenance ChecklistThe complete homeowner HVAC maintenance schedule: monthly, seasonal, and annual tasks that extend equipment life and cut energy bills.