What causes Carrier Gas Furnace 12?

A brief power interruption (outage, flickering power, tripped breaker, or someone switching the furnace off) during a heating cycle. Carrier's guide calls this normal operation. The heat call ending or being interrupted during the blower on-delay, for example the thermostat being turned down right after the burners lit (variable-speed models). Line voltage that is too low during a call for heat, including undersized wiring between the main panel and the furnace. Low 24-volt control voltage from a failing transformer, or a thermostat or thermostat wires that are loading the transformer down. A short in the humidifier solenoid coil, its wiring, or the diode bridge (if used). A short in the gas valve or the wiring to the gas valve. A direct short in a pressure switch wire (the low-heat or high-heat pressure switch circuit).

How do I fix Carrier Gas Furnace 12?

Confirm the code: look through the sight glass on the blower door and count 1 short flash then 2 long flashes. Write it down before turning power off. If it showed once after a power outage, a flicker, or someone using the furnace switch, wait out the 90-second blower run. Carrier says the furnace then responds to the thermostat normally. Check that the furnace power switch and its breaker are fully on, and that nothing (like a loose switch plate or a shared circuit tripping) is cutting power on and off. Make sure the blower door is installed and seated firmly. The door switch passes power to the control board, so a loose door can cut power mid-cycle. If the thermostat runs on batteries, replace them, and avoid turning the heat down and back up right after the burners light. If you have a furnace-mounted humidifier, note whether code 12 seems to appear when it runs. That's useful information for the technician, since Carrier lists the humidifier coil as a cause. If code 12 repeats every couple of minutes (inducer starts, stops, then the blower runs 90 seconds), call a pro. That points to a short Carrier's guide traces with live voltage tests.

When should I call a pro for Carrier Gas Furnace 12?

Code 12 keeps repeating every couple of minutes and the furnace never heats normally. The control's 24-volt supply needs checking (Carrier's guide checks R to COM-24V and the transformer), or the thermostat or its wiring is loading the transformer down. Line voltage at the furnace is low, or the wire gauge from the main panel doesn't match the installation instructions. The humidifier solenoid coil, its wiring, the gas valve, gas valve wiring, or pressure switch wires need to be tested for a short. This involves working inside a powered furnace with the door switch held closed. The gas valve needs to be tested or replaced. You smell gas: leave the house and call your gas utility. Never jumper or bypass a pressure switch or other safety device.

Code: 12🔥 Carrier Gas Furnace

Carrier Furnace Code 12: Blower On After Power Up (Meaning, Causes, Fixes)

Q: What does Carrier Gas Furnace 12 mean?

Status code 12 on a Carrier furnace (1 short flash, then 2 long flashes) means "blower on after power up": the furnace lost power, or the call for heat dropped out, during a heating cycle, so the control runs the blower for 90 seconds before going back to normal. Seen once after a power blip or a thermostat change, it's normal operation. If it repeats every couple of minutes, Carrier says it's probably a short in the pressure switch circuit, the gas valve or its wiring, or the humidifier coil, which needs a technician. Carrier status codes are two digits: the first digit is the number of short flashes and the second the number of long flashes, so code 12 is 1 short flash followed by 2 long flashes. On Carrier variable-speed (troubleshooting guide 58-06T) and 2-stage (guide 58-3T) furnaces, if power is interrupted during a call for heat (thermostat R-W/W1 closed) and the thermostat is still calling for heat when power returns, the control starts a 90-second blower-only period 2 seconds after power is restored and flashes code 12 during it. The variable-speed guide adds that it also happens if the call for heat is interrupted (R-W/W1 opens) during the blower on-delay period. After the 90 seconds, the LED goes back to steady on (if there are no other faults) and the furnace responds to the thermostat normally. Carrier's guides say that if code 12 repeats every couple of minutes, it is probably caused by a direct short in the pressure switch circuits, the gas valve, the wiring to the gas valve, or the humidifier coil. Low line voltage or a weak 24-volt supply (a failing transformer, or a thermostat or thermostat wiring loading it down) can cause the same pattern.

Reviewed 2026-10-23 · Based on 2 manufacturer & service sources

What the code means

Carrier status codes are two digits: the first digit is the number of short flashes and the second the number of long flashes, so code 12 is 1 short flash followed by 2 long flashes. On Carrier variable-speed (troubleshooting guide 58-06T) and 2-stage (guide 58-3T) furnaces, if power is interrupted during a call for heat (thermostat R-W/W1 closed) and the thermostat is still calling for heat when power returns, the control starts a 90-second blower-only period 2 seconds after power is restored and flashes code 12 during it. The variable-speed guide adds that it also happens if the call for heat is interrupted (R-W/W1 opens) during the blower on-delay period. After the 90 seconds, the LED goes back to steady on (if there are no other faults) and the furnace responds to the thermostat normally. Carrier's guides say that if code 12 repeats every couple of minutes, it is probably caused by a direct short in the pressure switch circuits, the gas valve, the wiring to the gas valve, or the humidifier coil. Low line voltage or a weak 24-volt supply (a failing transformer, or a thermostat or thermostat wiring loading it down) can cause the same pattern.

Likely causes

A brief power interruption (outage, flickering power, tripped breaker, or someone switching the furnace off) during a heating cycle. Carrier's guide calls this normal operation
The heat call ending or being interrupted during the blower on-delay, for example the thermostat being turned down right after the burners lit (variable-speed models)
Line voltage that is too low during a call for heat, including undersized wiring between the main panel and the furnace
Low 24-volt control voltage from a failing transformer, or a thermostat or thermostat wires that are loading the transformer down
A short in the humidifier solenoid coil, its wiring, or the diode bridge (if used)
A short in the gas valve or the wiring to the gas valve
A direct short in a pressure switch wire (the low-heat or high-heat pressure switch circuit)

Safe DIY checks

⚠ Before you start

Turn off power at the furnace/HVAC service switch and the breaker before opening panels. Shut the gas valve before any work near the burners, and never disconnect or alter the gas line. If you smell gas, leave the house and call your gas utility.

Confirm the code: look through the sight glass on the blower door and count 1 short flash then 2 long flashes. Write it down before turning power off.
If it showed once after a power outage, a flicker, or someone using the furnace switch, wait out the 90-second blower run. Carrier says the furnace then responds to the thermostat normally.
Check that the furnace power switch and its breaker are fully on, and that nothing (like a loose switch plate or a shared circuit tripping) is cutting power on and off.
Make sure the blower door is installed and seated firmly. The door switch passes power to the control board, so a loose door can cut power mid-cycle.
If the thermostat runs on batteries, replace them, and avoid turning the heat down and back up right after the burners light.
If you have a furnace-mounted humidifier, note whether code 12 seems to appear when it runs. That's useful information for the technician, since Carrier lists the humidifier coil as a cause.
If code 12 repeats every couple of minutes (inducer starts, stops, then the blower runs 90 seconds), call a pro. That points to a short Carrier's guide traces with live voltage tests.

🛠 When to call a pro

Code 12 keeps repeating every couple of minutes and the furnace never heats normally.
The control's 24-volt supply needs checking (Carrier's guide checks R to COM-24V and the transformer), or the thermostat or its wiring is loading the transformer down.
Line voltage at the furnace is low, or the wire gauge from the main panel doesn't match the installation instructions.
The humidifier solenoid coil, its wiring, the gas valve, gas valve wiring, or pressure switch wires need to be tested for a short. This involves working inside a powered furnace with the door switch held closed.
The gas valve needs to be tested or replaced.
You smell gas: leave the house and call your gas utility. Never jumper or bypass a pressure switch or other safety device.

Sources

Last reviewed 2026-10-23. Always confirm against the manual for your exact model number.

Carrier Furnace Code 12: Blower On After Power Up (Meaning, Causes, Fixes)

What the code means

Likely causes

Safe DIY checks

🛠 When to call a pro

Related codes

Sources