Quick answer. Check the power (breaker + disconnect), the thermostat, ice on the lines, the outdoor fan, the indoor blower, the filter, 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 wall light switch, usually mounted on or near the furnace (sometimes at the top of the basement stairs).
- Has a breaker tripped in your electrical panel — one for the furnace, and a separate one for the outdoor AC unit?
- Is the outdoor disconnect fully seated? That’s the small gray box mounted on the wall next to the outdoor unit, with a pull-out or hinged block inside it.
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. Look for ice
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. Running an AC iced up can damage the compressor, and it almost always points to low refrigerant or an airflow problem — this one’s a call-a-pro situation, not a DIY fix.
4. 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.
- Without touching anything electrical, you can feel the small copper line (the suction line) running from the outdoor unit back toward the house. On a working AC it should feel cool and active. If you don’t feel any refrigerant movement in that line, that also points to a problem in the outdoor unit.
5. Check the indoor unit
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The blower
- Is the indoor blower actually running? You should hear or feel air moving from the vents.
- 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.
6. Check the filter
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Clean & facing the right way
- Pull the filter and hold it up to a light. If you can’t see light through it, it’s overdue.
- Check the direction — there’s an airflow arrow printed on the filter’s frame, and it should point toward the furnace or air handler, not away from it. A backwards filter restricts airflow and can freeze the coil or overheat the furnace.
7. Check the vents
Make sure your supply vents and the return are open and not blocked by furniture, rugs, or curtains. Closing off too many vents restricts airflow and stresses the whole system — it’s one of the most common (and easiest to fix) causes of an unevenly heated or cooled home.
What this tells you
- No refrigerant movement in the small copper line — that’s an outdoor unit problem.
- Outdoor fan isn’t spinning — that’s also 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.
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.
Ready when you are
If you’ve been through the list and it’s still not working, give us a call. We run a full diagnostic for $80 — credited toward the repair — find the actual cause, and give you a flat price to approve before any work starts.
Still stuck? Let’s get it sorted
Tell us what you found on the checklist and we’ll take it from there — same-day and after-hours available.