On the covered Carrier gas furnace, 12 means normal operation. The blower runs for 90 seconds when power is interrupted and restored during a call for heat, or if the call for heat drops during the blower on-delay. Carrier says a 12 that repeats every couple of minutes is probably a direct short in the pressure-switch circuits, the gas valve, the wiring to the gas valve or a humidifier coil. First, note whether 12 appears once after a power flicker or repeats every couple of minutes.
Differential
Likely causes of “Carrier gas furnace code 12”, and how to tell them apart
- 1
Normal operation after a power interruption
What separates it Code 12 appears once and the blower runs for 90 seconds
How to confirm Nothing needs to be done
- 2
A call for heat that dropped during the blower delay
What separates it The code appears once
How to confirm Normal
- 3
A short in the pressure switch circuits
What separates it Code 12 repeats every couple of minutes
How to confirm Technician testing
- 4
A short in the gas valve or its wiring
What separates it The code repeats
How to confirm Technician testing
- 5
A humidifier coil short
What separates it The code repeats after the humidifier runs
How to confirm Technician testing
Before you touch a setting
Record this first — changing settings destroys the evidence
- That the code reads 12
- Whether it appears once or repeats
- Whether power flickered
- Whether a humidifier is fitted
Try in this order
0 of 8 completedWhat to do first
ReversibleNote whether 12 appears once after a power flicker or repeats every couple of minutes.
Correct only the documented fault domain
ReversibleOne 12 after a power interruption needs nothing. A repeating 12 is a wiring or component fault: Carrier's flowchart for it checks for less than 17 volts AC across R and COM-24V, less than 90 volts AC at L1 and neutral, and the wire gauge between the fuse box and the furnace, and ends at replacing the transformer, the thermostat wiring or the gas valve as the check indicates. That flowchart starts with removing the blower access panel, so it is technician work.
Note how often 12 appears
ReversibleOnce or repeatedly?
If once
ReversibleLet the blower finish.
If it repeats
ReversibleTurn the furnace off.
Check for a humidifier
ReversibleNote whether it is connected.
Record the pattern
ReversibleAnd the time.
Call a technician
ReversibleIf it repeats every few minutes.
Model-specific
What is true of this model and not of its siblings
- A code that is usually not a fault
- Carrier lists 12 as normal operation: if power drops and returns during a call for heat, the blower runs for 90 seconds to clear heat.
- When it is a fault
- A 12 that repeats every couple of minutes is probably a direct short in the pressure switch circuits, the gas valve, its wiring or a humidifier coil.
- Carrier's check
- The flowchart checks for less than 17 volts AC across R and COM-24V, and for less than 90 volts AC at L1 and neutral.
- What you can do
- Note the pattern. Do not open the control compartment.
- Humidifiers
- If a humidifier was fitted recently, tell the technician.
- Power quality
- Frequent flickers in the area are worth reporting to the utility.
- A normal code
- Many people worry about a code that is actually the control telling them it restarted. After a power flicker in the middle of a heating call, the blower runs for 90 seconds to clear heat from the heat exchanger. The display shows 12 during that time. Nothing is wrong.
- A repeating code
- If 12 repeats every couple of minutes, Carrier says the likely cause is a direct short in the pressure-switch circuits, the gas valve, the wiring to the gas valve or a humidifier coil. In that case the furnace keeps restarting, which can be seen as short bursts of the blower.
- Humidifier wiring
- Humidifiers are often wired into the furnace's 24 volt supply. A short in the humidifier's wire or coil can drag down the supply and trigger the code. If the code began after a humidifier was installed or serviced, tell the technician.
Stop boundary
When this stops being a self-service repair
- !Never bypass, jumper or tape over a safety switch
- !Stop for a carbon monoxide alarm, soot, scorch marks or flame outside the burner compartment
- !Call a qualified technician for anything behind the furnace's panels
- !If you smell gas, leave the building and call the gas utility from outside; do not use switches or phones inside
Evidence ledger
Sources behind this answer
Exact questions
Common questions
What does 12 mean on a Carrier gas furnace?+
Normal operation. The blower runs for 90 seconds when power is interrupted and restored during a call for heat, or if the call for heat drops during the blower on-delay. Carrier says a 12 that repeats every couple of minutes is probably a direct short in the pressure-switch circuits, the gas valve, the wiring to the gas valve or a humidifier coil.
What does code 12 mean on a Carrier furnace?+
Normal operation after power is interrupted and restored.
Do I need to do anything?+
Not for one 12.
What if it repeats every couple of minutes?+
Carrier says a short is probable.
Is a humidifier a suspect?+
Carrier lists a humidifier coil.