Quick answer. Check the power (switch, furnace door, disconnect, breakers), the thermostat, the condensate drain, ice on the lines, the outdoor fan, the blower, and the vents. If everything checks out and it’s still not working, that’s your sign to call — not a sign you missed something.
1. Check the power
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Furnace & AC power
- Is the furnace’s power switch on? It looks like a normal light switch, usually mounted on or near the furnace.
- Is the furnace door on correctly? Furnaces have a door safety switch. A loose or missing door opens that switch, and the furnace won’t run.
- Check the outdoor unit’s power at the disconnect, the small gray box with an electrical whip running from it to the outdoor unit. Make sure it’s closed and hasn’t taken on water or anything else.
- Check the breakers for the A/C and the furnace separately. A tripped breaker usually means an electrical problem somewhere. Call and have someone find out exactly why it tripped.
2. Check the thermostat
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Settings & batteries
- Set to Cool (for AC) or Heat (for a furnace) — and is the set temperature actually past the current room temperature?
- Screen blank or dim? That usually means dead batteries, or the thermostat itself has lost power.
- Give it up to 5 minutes after you change the setting. Most systems have a built-in delay before the outdoor unit kicks on to protect the compressor — that’s normal, not a problem.
3. Check the condensate drain
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Drain, float switch & condensate pump
- Find the condensate drain coming off the furnace or the indoor evaporator coil, and look for a float switch (an overflow safety switch) or a condensate pump next to it.
- Have a float switch? Pull the top off it. Water inside means the drain is clogged.
- Clogged drain with a properly installed P-trap that has a cleanout? Open the cleanout and clear the trap. That alone can get the system running again.
- Have a condensate pump that isn’t running? A bad pump shuts the system down, and that one’s a call.
4. Look for ice
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Line set & indoor coil
- Check the indoor coil and the refrigerant lines running out to the outdoor unit. Ice anywhere — inside or out — means stop.
- Switch the thermostat to Off or Fan Only and let it thaw. Running an A/C iced up can damage the compressor.
- Ice means an airflow problem or something in the outdoor unit not working right. Check the air filter first. Pull it and hold it up to a light. If you can’t see through it, that’s your airflow problem.
5. Check the outdoor unit
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The condenser
- Is the outdoor fan spinning? If it’s not moving at all, that’s an outdoor-unit problem — often a bad capacitor, contactor, or motor. Don’t keep flipping the breaker to try again.
- Protect it: if the outdoor fan or compressor isn’t running, pull the disconnect or set the thermostat to Fan Only. The house keeps getting airflow, and the outdoor unit stops trying to run until someone gets there.
- Without touching anything electrical, feel the large insulated line running from the outdoor unit toward the house. On a working A/C it’s cool, often sweating. The small bare line beside it runs warm, and you should be able to feel refrigerant moving through it. Neither line doing anything points to the outdoor unit.
6. Check the airflow
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The blower
- Is air moving from the vents? Put a hand at the furnace cabinet when the system should be running. Can you feel the blower moving air?
- Blower running but the house still isn’t cooling or heating? Check the air filter. If you can’t see light through it, replace it.
- Blower running but the outdoor fan isn’t, or you can’t feel the compressor running? That’s a call to us or to an HVAC contractor. Until then, set the thermostat to Fan Only or pull the disconnect.
- Blower not running at all? Turn the system Off at the thermostat so it stops trying. A furnace without its blower can overheat, and an A/C without airflow freezes its coil.
- Hear a hum or a click but nothing starts? That’s usually electrical — a capacitor or contactor. Don’t keep trying; repeated attempts can make it worse.
7. Check the vents
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Supply vents, the return & dampers
- Make sure your supply vents and the return are open and not blocked by furniture, rugs, or curtains.
- Check for dampers in the ductwork. They’re either electric dampers or a small metal handle on the duct, and that handle can work itself closed.
- Closing off too many vents restricts airflow and stresses the whole system.
What this tells you
- Water in the float switch, or a clogged trap — that’s a drain problem. A trap with a cleanout you can clear yourself; a dead condensate pump is a call.
- Ice on the lines with a clean filter — the airflow problem is past the filter, or it’s the outdoor unit. Let it thaw and call.
- Outdoor fan isn’t spinning, or no refrigerant movement in the lines — that’s an outdoor unit problem.
- Everything above checks out and it’s still not working — that’s exactly when it’s time to call. You haven’t missed anything; the problem is just past what you can safely check from the outside of the equipment.
Whatever you find, shut the system off the way that step says. A system that keeps trying to run with a dead part does more damage by the day, and that turns a small repair into a bigger one.
Ran the checklist and it still won’t run?Call or Text 801-960-5925
Call a professional right away — skip the checklist
- You smell gas. Leave the house, call the gas company or 911 from outside, then call us.
- A breaker keeps tripping after you reset it. That’s usually an electrical short, not a fluke — leave it off.
- Water is pooling near the furnace or indoor coil — likely a clogged condensate line or a tripped safety float switch.
- Anything involving refrigerant. It’s federally regulated (EPA) and requires a licensed, certified technician to handle — that’s not a DIY step.
Want more detail on your specific problem?
This is the general run-through. For a deeper walkthrough on one system: furnace won’t turn on — 7 things to check or AC not cooling — 7 things to check.
Still stuck? Let’s get it sorted
Tell us what you found and we’ll take it from there. The diagnostic is $80, credited toward the repair.