Folder · 5 symptoms
When the AC or Furnace Won't Run Right
AC and furnace troubleshooting starts with the thermostat, the breaker and switches, and the filter; most no-cool, no-heat and stop-start problems show up in those three before a technician is needed.
Checked against makers' and agencies' documents by Hum Drip Hiss · how pages are made · Updated
Vitals
- For an AC that quits, Carrier's first checks are the breaker panel, the thermostat and the filter: 3 stops before any call.
- A thermostat fan left on ON is the most common reason a furnace blows cold air, according to Carrier.
- Reset a tripped HVAC breaker once; if it trips again, leave it off and call a licensed technician.
- Short cycling means repeated cycles of only a few minutes; makers use roughly 5–10 minutes as the line.
- Furnace blink codes differ by brand and model, so match the count to the label on your own furnace's door.
- Bryant says safe homeowner troubleshooting ends before the equipment cabinet is opened.
Where does AC and furnace troubleshooting start?
It starts at the thermostat, then power, then the filter: Carrier's first three furnace checks are Heat mode, the furnace power switch and the air filter, and its AC list swaps Heat for Cool. These take about 10 minutes and need no tools. Check the mode, the fan setting (AUTO, not ON) and the batteries; look for a tripped breaker, the outdoor disconnect and the furnace switch; then pull the filter. The safe checks list runs through each one and where it ends.
Start with these
Why won't the AC cool or turn on?
An AC that won't cool usually has a setting, airflow or power problem, and Lennox notes that when the outdoor unit has lost power the indoor fan just moves warm air around. If nothing runs at all, the thermostat, breaker, disconnect or a float switch in the drain pan is the likely stop. Ice on the coil and low refrigerant come after those; refrigerant is only for EPA Section 608 certified technicians.
Cooling pages
Why won't the furnace heat?
A furnace that won't heat is either blowing room air, shutting itself down on a safety switch, or failing to light. A fan on ON tops Carrier's list of reasons for cold air from a furnace, and Carrier adds that a clogged filter can trip the limit switch so the burners stop while the fan keeps running. If the burners never light, the furnace usually shows a blink code and may lock out; the igniter, flame sensor and gas valve are a technician's work.
Heating pages
What does short cycling mean for the AC or furnace?
Short cycling means the system starts and stops over and over in cycles of only a few minutes; Carrier describes furnace cycles often less than 5 to 10 minutes. A dirty filter, a thermostat in direct sunlight, an overheating furnace or an oversized system are the usual causes. On a very cold night, Trane notes, more frequent furnace cycles can be normal.
Stop-start pages
Which AC and furnace faults need a technician?
Anything behind a cabinet panel does: Bryant warns against working on capacitors, compressors, contactors or refrigerant systems, and Carrier says bypassing or removing the flame sensor is dangerous. That includes igniters, gas valves, limit and rollout switches, control boards and any refrigerant work. Write down what you saw and checked; the home page lists every other symptom on this site.
Stop and call if
- Gas smell: everyone out of the house, then phone 911 or the gas company from a safe distance.
- The HVAC breaker won't stay on after a single reset.
- The outdoor fan hums but won't spin, or you smell burning plastic or wiring.
- A bang or boom sounds as the burners catch.
- A blink code or lockout comes back after one power cycle.
- If you're unsure, stop and call a licensed HVAC technician.
Readings
- 3first checks for a dead AC: breaker, thermostat, filterCarrier
- 1breaker reset allowed before callingLennox
- 5–10 mincycle length Carrier uses as the short-cycling lineCarrier
- 30–60 sblower delay after ignition on some furnacesCarrier
| Symptom | First safe check | Most likely cause | Page |
|---|---|---|---|
| AC blows warm air | Mode COOL, fan AUTO | Fan setting or outdoor unit off | AC blowing warm air |
| AC does nothing | Batteries, breaker, disconnect | No power or float switch | AC not turning on |
| Furnace blows cold air | Fan AUTO, new filter | Fan on ON or limit trip | Furnace blowing cold air |
| Furnace never lights | Switch, door panel, blink code | Ignition failure or lockout | Furnace won't ignite |
| Runs a few minutes, stops | Filter, thermostat location | Overheating or oversizing | Short cycling |
Case notes
Behind this hub: the five pages in the folder, which rest on Carrier, Lennox, Trane and Bryant troubleshooting pages, Lennox and Carrier furnace manuals, and EPA and ENERGY STAR pages, all read in October 2026. Figures here match those pages. No cost figures, no repair steps inside any cabinet, and no universal blink-code table.
Asked at intake
What should you check first when neither heat nor AC works?
Check the thermostat screen and batteries, then the breaker and the furnace or air handler switch, since one thermostat and, in most homes, one indoor blower serve both. If the thermostat is blank, a tripped float switch in the drain pan can also cut power.
Can one dirty filter cause both heating and cooling problems?
Yes. In cooling it can help freeze the coil, and in heating Carrier says it can trip the limit switch so the burners shut off. Check it monthly and replace it at least every 3 months for 1-inch filters.
Is it safe to turn the system off and on to reset it?
Once, yes. Carrier's AC reset waits at least 30 minutes with the breaker off. Repeated power cycling to force a start is not a fix, and a breaker that trips twice should stay off.
Why does the system run but the house never reaches the setpoint?
A dirty filter, blocked returns, a dirty coil or low refrigerant can all do it. Start with the filter and open registers, then call if it still runs nonstop.
Do heat pumps have their own won't-run problems?
Yes. Defrost cools the supply air briefly, usually a few minutes and at most about 15. AUX heat helps during defrost and in deep cold; call if it keeps running for long stretches when it's mild out.
When is it an emergency rather than a repair?
A gas smell, a sounding CO alarm or smoke means get everyone outside first and call 911 from there. A burning plastic smell means shut the system off at the thermostat and switch and call. The stop-and-call page lists each sign.
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