The short answer
Code 12 on a Carrier furnace in Northern Virginia usually isn’t a problem. It means the board got power while the thermostat was already calling for heat, or the call ended during the blower delay, so it runs the blower for about 90 seconds before it starts over. After a power blip, that’s normal. If 12 keeps coming back every couple of minutes, it’s pointing at a short in the circuit, and that’s a call.
How to read code 12
Look through the small window in the lower door. 12 is 1 short flash and 2 long ones, then a pause before it repeats. On Carrier’s two-stage and variable-speed boards the light is amber, and stored codes erase themselves after 72 hours. Newer display boards, like the one on Bryant’s 916SA, show it as 12.1. Our full list of Carrier furnace codes has every one.
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What Carrier’s guide says it means
Carrier’s troubleshooting guide for its 58-series variable-speed, two-stage furnaces says the blower runs for 90 seconds if the furnace is powered up during a heat call, meaning the thermostat’s R to W (or W1) circuit was already closed, or if the heat call opens during the blower on-delay. Once that’s done, the furnace goes back to normal.
When it’s normal
- Right after a power outage or a flicker, if the thermostat was calling for heat.
- After someone turned the furnace switch off and on during a heat call.
- When the thermostat stopped calling for heat right after the burners lit, before the blower had started.
In each case the blower runs about 90 seconds, the light goes back to steady, and the next heat call runs normally.
When it’s a problem: 12 over and over
Carrier’s guide says if code 12 repeats every couple of minutes, the likely cause is a direct short somewhere on the circuit: the pressure switch circuit, the gas valve, the gas valve wiring, or a humidifier coil. The pattern looks like this: the inducer starts, the inducer stops, the blower runs for 90 seconds, and it starts over without ever lighting. A short can also blow the 3-amp fuse on the board, which flashes 24. Our Carrier code 24 guide covers that one.
What you can check
- Write down the code and watch one full cycle. Does it light and heat, or does it loop?
- If you have a whole-house humidifier on the furnace, note it when you call: its valve is on Carrier’s list.
- Make sure the lower door is on tight and the furnace switch is on.
- If it loops more than a few times, turn the furnace off at the switch and call. A short on the gas valve circuit isn’t something to leave running.
What a technician checks
We measure the 24-volt control power between R and COM, and the line voltage coming in. Carrier’s guide flags under 17 volts on the control side, or under 90 on the line side. Then we take each part the guide names off the circuit, one at a time, until the short shows itself, and you get the price of the fix before we make it.
Codes next to 12
Code 11 means no previous code is stored, so nothing’s wrong. Code 13 is a limit lockout: a limit, draft safeguard or rollout switch stayed open more than 3 minutes, or a blocked vent kept it open, or the limit tripped 10 times in high heat. The board resets itself after 3 hours. Our Carrier code 33 guide covers 13 too.
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.
- Every Carrier furnace code and what to do
- Carrier code 31
- Carrier code 33 and 13
- Carrier code 24
- How to reset a Carrier furnace
- Error codes and warning lights by brand
Every code meaning here comes from Carrier’s own documents: 58-series variable-speed two-stage furnace troubleshooting guide.
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 12 mean on a Carrier furnace?
The board got power during a heat call, or the call ended during the blower delay, so it runs the blower for about 90 seconds before it starts over. After a power blip, it’s normal.
Is code 12 dangerous?
Once, no. If it repeats every couple of minutes and the furnace never lights, Carrier’s guide points at a short on the pressure switch circuit, the gas valve or a humidifier coil. Turn it off and call.
Why does my furnace keep flashing code 12?
If it comes back again and again, it’s usually a direct short in the low-voltage circuit. The inducer starts and stops, the blower runs 90 seconds, and it loops without lighting.
How do I clear code 12?
You don’t need to. It clears after the 90-second blower cycle, and stored codes on Carrier’s amber boards erase themselves after 72 hours.
How much does it cost to have you look at it?
A repair visit starts with a flat $99 diagnostic, and you get the price of the fix before any work starts. Our heating check is $67 per system, through December 31.
