HVAC troubleshooting: what to check before you call anyone
Six symptoms cover most of the calls a heating and cooling company gets. Each one below names what usually causes it, the checks that are genuinely safe to run yourself, and the point where a licensed technician takes over. Nothing here asks you to open a gas valve or handle refrigerant.
Start with the symptom you actually have
Tap a row to open itNo fan, no click, nothing. Before you assume the furnace is dead, check what is asking it to run. A thermostat still in cool mode, a bumped service switch on the side of the cabinet, or a breaker that tripped during the last storm all leave a perfectly good furnace silent. After three failed ignition attempts most control boards lock themselves out for an hour or more, so a furnace that did nothing at 6am may simply be waiting.
- 01Thermostat in Heat, setpoint above room temperature, fan on Auto
- 02The breaker at the panel and the service switch on the furnace cabinet
- 03The filter — a choked filter is enough to stop some units accepting a call for heat
If the fault light returns to the same code after one power reset, stop resetting it and call a licensed technician.
Most common: thermostat not in Heat, or a tripped service switch.
The burner lights, runs a few seconds, then drops out. That pattern usually points at the safety circuit rather than the flame: a flame sensor coated in oxide stops proving flame, an inducer that can't build enough draft trips the pressure switch, or a flue and intake plugged by snow, ice or a nest does the same. A full condensate trap will shut a high-efficiency furnace down too, often on the first really cold day of the year.
- 01Clear snow, ice, leaves or nests from the two white pipes outside
- 02Empty the condensate trap if the furnace has one
- 03A fresh filter, and every supply register open
Never bypass a pressure switch or tie a sensor back to keep the burner lit — both exist to stop unburned gas collecting in the cabinet.
Nine times in ten it is the flame sensor.
The panel is on, the thermostat shows heat, and still nothing moves. On most furnaces the blower door must seat a safety switch before the sequence will start at all, so a panel that didn't click back into place looks exactly like a dead furnace. If the door is seated, the control board's light is the fastest read you will get: a repeating blink pattern is a fault code, and the legend for it is printed on the inside of that same door.
- 01Both panels fully seated and screwed down
- 02Thermostat batteries, or the app if it is a smart thermostat
- 03The LED blink pattern, noted down before you touch anything else
No light at all with the power on means the board is not getting power — that is a technician's diagnosis, not a parts guess.
Start with the blower door switch.
You can hear the sequence start, but no flame arrives. On a modern furnace a hot-surface igniter glows orange just before the gas valve opens; a cracked igniter, a gas valve that never opens, or a shutoff handle left across the pipe will all stop it right there. On an older standing-pilot furnace the thermocouple is the usual suspect. Check the gas first: if the water heater and the range are out too, the problem is the supply, not the furnace.
- 01The gas shutoff handle — parallel to the pipe means open
- 02Whether the other gas appliances in the building are working
- 03Whether the igniter glows through the sight glass, without opening anything
Do not relight a pilot or open the burner compartment on a sealed-combustion furnace. If you smell gas, leave the building and call your gas utility.
Handle parallel to the pipe means the gas is on.
A blower running while the air stays cold is usually the furnace protecting itself, not a fan fault. The high-limit switch opens when the cabinet gets too hot — most often a filter nobody has changed — and the board responds by running the blower to cool things down. The same symptom shows up when the igniter has failed and the board runs the fan on its own schedule rather than sit idle. Either way, the fan is only the messenger.
- 01Filter condition, and every return grille clear of furniture or rugs
- 02All supply registers open — closing off half the house overheats the cabinet
- 03Whether the blower runs constantly or in short bursts, which changes what the code means
A limit switch that trips twice with a clean filter needs a technician; repeated lockouts can distort a heat exchanger.
Cold air from the vents is usually a limit trip.
Warm air with the fan running is the most common cooling complaint, and the outdoor unit is usually where the answer is. If the condenser fan is not spinning or the compressor is silent, nothing is being cooled no matter what the thermostat says. A thermostat with the fan set to On instead of Auto will also move air all day without ever cooling it — and that one costs nothing to fix.
- 01Thermostat in Cool, setpoint below room temperature, fan on Auto
- 02The outdoor unit actually running — fan spinning and compressor humming
- 03Two feet of clear space around the condenser, and a clean filter inside
Refrigerant is a sealed system: a low charge means a leak, and topping it up without finding the leak just buys the same call next summer.
Check the outdoor unit first, every time.
Where troubleshooting stops
If you smell gas, hear a boom, see soot or a scorched panel, or the furnace trips a limit switch twice with a clean filter — stop and call a licensed technician or your gas utility. Leave the building if you smell gas. Refrigerant, gas piping and anything inside a live electrical panel are not owner-serviceable, whatever a video suggests.
Photograph the nameplate before you call
Brand, model, serial and capacity are what every technician quotes from, and they decide whether a part is even still made. One photo of the data plate puts the unit on record, and HVAC Scout reads the specs, the age and the likely causes from there.
Start with a free scanQuestions owners ask here
Why is my furnace running but blowing cold air?
Usually a restricted filter or closed registers pushing the high-limit switch open, so the blower runs to cool the cabinet while the burners stay off. A failed igniter produces the same cold-air symptom. Change the filter, open the registers, and note the fault light before calling anyone.
How many times should I reset my furnace?
Once. If it locks out again after a single reset at the breaker or the board, stop — repeated resets on a fuel-burning appliance can hide a venting or gas problem that a technician needs to see.
What does the blinking light on my furnace mean?
It is a fault code. The legend is printed on the inside of the blower door on almost every residential furnace: count the blinks, note any pause pattern, and compare it to that sticker. The same code tells a technician where to start.
Can I check the gas valve myself?
Only the handle. A handle parallel to the pipe is open, across the pipe is closed. Anything past that — testing voltage at the valve, opening the burner compartment, relighting a pilot — belongs to a licensed technician.
Why isn't my AC cooling when the fan runs?
Most often the outdoor unit isn't running at all, the thermostat fan is set to On rather than Auto, or the filter and coil are loaded with dust. If the outdoor fan and compressor are both running and the air is still warm, the system is likely low on refrigerant from a leak.
How do I find my model and serial number for a technician?
It is on the data plate — a metal or printed label on the cabinet, usually on the outer panel or inside the blower door. Photograph it rather than copying the digits by hand; HVAC Scout reads the brand, model, capacity and age straight from the photo.