Bryant 355MAV Error Code 12: Blower On After Power Up
What Does Code 12 Mean?
Status code 12, "Blower On After Power Up," is a normal operating note on the Bryant 355MAV rather than a fault. It records that the variable-speed blower ran for roughly 90 seconds immediately after power was restored.
This happens when the furnace is powered up (115 VAC or 24 VAC) while the thermostat is already calling for heat with the R to W/W1 circuit closed, or if that heat call opens during the blower on-delay period. The board runs the blower briefly as a safe, controlled response to being energized mid-call, letting it initialize and clear residual heat before starting a normal ignition sequence.
Because it describes designed behavior, code 12 needs no repair. You are most likely to see it after a power outage, after resetting the furnace breaker, or after the blower door switch is reengaged. Once the 90-second run completes, the furnace should proceed into its normal heating cycle.
What You'll Notice
- The blower runs on its own for about 90 seconds right after power is restored, before the burners light
- It typically follows a power outage, a breaker reset, or reseating the blower door while the thermostat is set to heat
How This Is Diagnosed
No diagnosis is required, but the context is easy to confirm: code 12 lines up with a recent power-up event while the thermostat was calling for heat. If the blower's brief run is followed by normal ignition and steady heating, the code has served its purpose and can be ignored. It only warrants a closer look if the furnace fails to move on to a normal heating cycle afterward, in which case any accompanying fault code — not code 12 — is the one to investigate.
- The blower runs the 90-second post-power-up cycle but the furnace then fails to ignite or heat normally, pointing to a separate fault code
- The blower runs continuously and never stops rather than for the brief post-power-up period
Frequently Asked Questions
Why did my Bryant 355MAV blower turn on by itself after a power outage?
That is code 12, a normal response. When power returns while the thermostat is calling for heat, the board runs the blower for about 90 seconds before proceeding with its normal heating sequence.
Do I need to fix code 12?
No. It is an informational status describing designed behavior, not an error. As long as the furnace goes on to heat normally, no action is needed.
Sources
✓ Verified against manufacturer service manual — March 2026