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Carrier · Furnace

12.1 W (heat call) on at power up

Low severity 59SC6A

What the control is telling you

The control saw a heat call (W) when power came on, typically after a power interruption during a heat call. About 2 s after power returns it runs the blower alone for 90 s (LED flashes 12, display 12.1), then resumes normal operation.

Applies to: Carrier 59SC6A

Likely causes

  • Power outage or breaker trip during a heat call
  • Blower door opened (interlock switch ILK) during a heat call
  • Loose line-voltage connection causing a momentary power loss

Test and repair steps

  1. Wait out the 90-second blower-only period; after it the LED should go to steady ON if no faults are present and the furnace should answer the thermostat normally.
  2. If 12.1 keeps appearing, check FLt history for repeated power-related codes (12.1, 46.1).
  3. Warning: Turn off and lock out power before checking line-voltage connections.
  4. Check the junction box and disconnect/breaker connections for looseness.
  5. Make sure the blower door is installed securely so the door interlock switch stays closed.
  6. Confirm the thermostat W wire is not intermittently shorting to R, which can look like a heat call at power-up.

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Source: Carrier 59SC6A Installation, Start-up, Operating and Service Instructions, Fig. 60 Service Label p.54; Sequence of Operation note, p.68. Summarized in our own words; check the original manual for your model.

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For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.