Most HVAC trouble shows up as one of a few symptoms: the system will not start, it starts and stops too often, it runs without heating or cooling, it ices over, or the air smells or feels wrong. The guides below start from the symptom, explain the likely causes, and separate what you can safely check from what needs a technician.
Safety first: gas and carbon monoxide
- Gas smell (like rotten eggs): leave the house at once. Do not switch lights or appliances on or off, and call the gas company or 911 from outside.
- Carbon monoxide alarm: get everyone out into fresh air and call 911. Headaches, dizziness or nausea in several people at once are a warning sign even without an alarm. Keep CO alarms working near the bedrooms.
- A burning or electrical smell, or a breaker that trips again after one reset: turn the system off and call. These calls are taken 24/7 through emergency HVAC.
- A furnace that locks out after trying to light is protecting itself. Resetting it again and again will not fix the cause.
Safe checks and technician work
| Area | Safe for you | Leave to a technician |
|---|---|---|
| Thermostat | Mode, setpoint, batteries, schedule | Wiring, compatibility, a missing C-wire |
| Airflow | The filter, open registers and returns | Blower motors, duct problems |
| Power | One breaker reset, the furnace service switch | Capacitors, contactors, control boards |
| Outdoor unit | Clearing plants and debris | Refrigerant, the compressor, the fan motor |
| Ice on coils or lines | Switching the system off so the ice can melt | Finding why it froze: airflow, charge or defrost |
The guides
Air conditioning. AC not turning on works from the thermostat and breakers to the fuse, the capacitor, the contactor and a drain safety switch. AC short cycling explains why a system that starts every few minutes wastes energy and wears its compressor, and when oversizing is the cause.
Furnaces. Furnace not heating separates cold air, no air and heat that is weak or comes and goes. Furnace not igniting covers igniters, flame sensors, the gas valve, older pilot-light systems and the control board. Furnace short cycling looks at a clogged filter, overheating, a dirty flame sensor, the thermostat and an oversized furnace.
Heat pumps. Heat pump freezing up explains which frost is normal defrost and which ice means a fault.
Controls and air. Thermostat not working covers blank screens, wrong readings and systems that ignore the setting. Poor indoor air quality covers dust, allergies, musty odors, mold and humidity.
Other symptoms
An AC that runs without cooling, or water around the indoor unit, belongs to AC repair. New grinding, screeching or banging means switching off and calling. Rooms that never match the thermostat usually point to airflow or the ducts, and a mini-split can serve a room the ducts never reached. Bills that climb without a reason call for a tune-up. A dusty smell on the first cold morning usually fades quickly; a burning-plastic smell does not, so turn the system off.
In San Jose, AC calls cluster on the first hot afternoons of the year and furnace calls on the first cold nights of fall. A seasonal check before each catches most of these problems early.