LED flashes code 12; blower runs briefly at power-up, then furnace may or may not start normally.

Note: On many Comfortmaker furnaces using ICP/Comfortmaker control boards (e.g., HK42FZ, CEPL130438), code 12 can indicate a normal blower-on after power-up or a stored previous blower run condition.

Steps:

• Allow furnace to complete a cycle: After power is restored, the blower may run for 60–90 seconds. This is normal.
• Check for active faults: After a few minutes, verify whether another code appears (e.g., 31, 33, 34). If so, troubleshoot that code.
• Inspect filter and airflow: If the blower runs unusually long at power-up, check the return air filter and supply registers for blockage that could cause residual heat.
• Reset power: Turn off power to the furnace for 30 seconds, then restore. Observe whether code 12 clears and normal operation resumes.

When to call a technician: If the furnace never proceeds beyond code 12 and does not attempt ignition, the control board or thermostat wiring may need professional diagnosis.