The short answer
Code 24 on a Carrier furnace in Northern Virginia means the fuse on the control board that protects the 24-volt wiring has blown. Something shorted that low voltage circuit: a thermostat wire rubbed bare, a nicked wire running out to the air conditioner or heat pump, or a new thermostat wired with the power on. The thermostat, the furnace controls and the outdoor unit all run on that one circuit, so everything quits at once, heat and cooling both.
What you’ll notice
- The thermostat goes blank, or it calls for heat and nothing happens.
- The AC or heat pump outside won’t run either.
- The light behind the furnace window flashes 2 short and 4 long.
- It started right after a thermostat swap, yard work near the outdoor unit, or a storm.
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How to read the code
Look through the small window in the lower door. Short flashes are the first digit and long flashes are the second, so 24 is 2 short and 4 long, then a pause before it repeats. A steady light is normal, and rapid flashing means the line voltage polarity is reversed. On older boards with a red light, cutting the power erases the code, so count it before you flip the switch. Newer Carrier boards keep the last 7 faults through a power cut and print a code with a third digit, like 13.1, on a small display. Our full list of Carrier furnace codes has every one.
What causes code 24
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| Where the short is | What usually led to it |
|---|---|
| At the thermostat | A new or smart thermostat wired with the power on, or two bare wire ends touching on the wall plate |
| The thermostat wire in the wall | A nail or screw through it, or a wire chewed or pinched |
| The wire to the outdoor unit | A mower, trimmer or shovel nicked it where it runs along the house |
| A part on that circuit | Something wired into the 24 volts shorted inside, like a coil or a safety switch |
Carrier’s own step for a 24 is to find the short by disconnecting the thermostat leads at the board and seeing whether the fault clears. Then each wire goes back on, one at a time, until the short shows itself.
What you can safely check
- If you just changed thermostats, turn the furnace power off and look at the wall plate. Bare copper touching another wire, or a wire on the wrong terminal, is the likely culprit. Our thermostat troubleshooting guide helps.
- Walk outside and look at the thin wire running to the outdoor unit. Cuts and bare spots near the ground are common after yard work.
- Turn the power off and back on once. If 24 comes right back, the short is still there.
What not to do
Don’t drop a new fuse in without finding the short. The new one blows too, sometimes the moment the thermostat calls. And never put in a bigger fuse or wire around it: that fuse is what stands between a short and the control board.
What a technician does
We pull the thermostat and outdoor wires off the board, test each run for a short to ground with a meter, find the bad one, repair or replace that wire or part, and put in the right fuse. Then we run heat and cooling to prove both work.
What it costs to have us look
A repair visit starts with a flat $99 diagnostic, and you get the price of the fix before we touch anything. Our Carrier repair page lists what the common repairs cost. Our heating check is $67 per system, through December 31: furnace, heat pump or boiler.
Related: thermostat not working, furnace won’t turn on, the full Carrier code list and our Carrier repair page.
Every code meaning here comes from Carrier’s own documents: 58SC0A installation instructions and 59MN7C manual.
Call or book
Call (703) 997-9757 and tell us the code you counted, or book online from goswiftpro.com/cookiesoffer and a box of warm cookies comes with the visit. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276.
Questions people ask
What does code 24 mean on a Carrier furnace?
The fuse on the control board that protects the 24-volt wiring blew, because something on that circuit shorted.
Why did my air conditioner stop working with code 24?
The thermostat, the furnace controls and the outdoor unit all run on the same 24-volt circuit. When its fuse blows, all of them stop.
Can I just replace the fuse on my Carrier furnace?
Not until the short is found. A new fuse on a shorted circuit blows again, and a bigger fuse can ruin the control board.
Can a new thermostat cause code 24?
Yes. Wiring a thermostat with the power on, or letting two bare wires touch, shorts the circuit and blows the fuse.
