12
Carrier Blower On After Power Up

Furnace attempts to start but stops, LED shows code 12 (blower on after power up).

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Safety Warning

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

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Possible Causes

Recent power interruption during a heat cycle, control board fan off-delay, stuck blower relay on control board, miswired G (fan) signal

How to Fix / Troubleshooting

Safety first: Turn off power before touching wiring or the control board.

Code 12 often appears briefly after power is restored and is not necessarily a fault.

  • Wait for normal timeout: If the blower is running, allow 60–90 seconds to see if it shuts off by itself. This can be normal post-purge operation.
  • Check thermostat fan setting: Ensure the thermostat fan mode is set to AUTO, not ON. A fan set to ON will keep the blower running continuously.
  • Inspect G terminal: With power off, verify that only the thermostat green wire is connected to the G terminal on the Carrier control board. Remove any jumper between R and G if present.
  • Check for stuck relay: Restore power. If the blower runs immediately with no call for heat or fan, the blower relay on the control board may be stuck closed, requiring board replacement.

If the blower never shuts off or the code persists with no call for heat, contact a professional to test the control board and low-voltage circuits.

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Repair Difficulty

Medium 3/5

Required Part

Furnace Control Board
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