Bryant 340MAV Error Code 12: Blower On After Power Up
What Does Code 12 Mean?
Status code 12 on the Bryant 340MAV is displayed when the circulating blower runs immediately after the furnace is powered on. This occurs under two specific conditions: the unit was powered up while a call for heat was already active (R-W/W1 thermostat contacts closed), or the R-W/W1 contacts opened during a blower on-delay period.
The blower run is a protective measure in the Carrier integrated furnace control. When power is restored during an active heating call, the control cannot immediately verify whether the heat exchanger is already hot from a previous cycle before the outage. Running the blower first clears residual heat and lets the board start its checks from a known thermal state before attempting ignition.
This code clears on its own once the blower cycle completes and the furnace proceeds through its normal startup sequence. It does not indicate a fault, and it is unrelated to the limit-driven blower cooling you would see with codes 33 and 13, which happen because of overheating rather than a power-up event.
What You'll Notice
- The circulating blower runs by itself for a short period right after power is restored, without a fresh thermostat call
- After that brief blower run the furnace proceeds to its normal startup and code 12 stops being displayed
How This Is Diagnosed
Code 12 is identified by timing: the blower-only run happens immediately after power-up and is a fixed protective interval, not a response to a live heat call. A technician confirms it is benign by verifying the furnace then completes a normal ignition sequence. If instead the blower runs continuously or never hands off to heating, the check moves to the thermostat wiring (R-W) and the blower on/off delays rather than treating code 12 as the fault.
- The furnace does not begin a normal heating sequence after the power-up blower cycle completes
- The blower runs continuously and never transitions into a heating cycle
Frequently Asked Questions
Why does my Bryant 340MAV blower kick on after a power outage?
If power returns while the thermostat is still calling for heat, the board runs the blower first to start from a known cool state. It is normal protective behavior recorded as code 12.
Is code 12 a problem I need to fix?
No. It is an informational status, not a fault. No action is needed as long as the furnace heats normally afterward.
✓ Verified against manufacturer service manual — March 2026