Carrier 59SC2B Error Code Rapid Flashing: Reversed Line Voltage Polarity
What Does Code Rapid Flashing Mean?
A rapidly flashing amber LED on the Carrier 59SC2B means the 115-volt power supply has reversed polarity — the hot and neutral wires are connected backwards. The control board detects this at startup and will not allow normal furnace operation until the wiring is corrected.
Reversed polarity can affect the furnace's safety systems, particularly the flame sensing circuit, which relies on proper grounding and polarity to accurately detect the presence of a burner flame. If the furnace is part of a twinned installation (two furnaces sharing ductwork), polarity issues can also arise from transformers being out of phase — the twinning kit instructions should be consulted in that case.
This is strictly an electrical wiring issue. The furnace hardware is not damaged. A licensed electrician or HVAC technician needs to trace the power supply wiring from the circuit breaker panel to the furnace and swap the hot and neutral connections to their correct positions. Once polarity is corrected, the furnace will resume normal operation.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Hot and neutral wires reversed at furnace connection | Most common | ✗ Call a pro → |
| Twinned furnaces with transformers out of phase | Uncommon | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- Immediately when this code appears — correcting line voltage polarity requires a licensed electrician or HVAC technician
✓ Verified against manufacturer service manual — March 2026