Carrier Furnace Rapid Flash
Hard lockoutShown as: Very fast continuous flashing with no pause between flashes
Line voltage (115 VAC) polarity is reversed at the furnace. The control cannot prove flame with reversed polarity, so the furnace will not run.
Manual wording: “RAPID FLASHING - Line voltage (115VAC) polarity reversed. If twinned, refer to twinning kit instructions.”
Documented for: 58MCB, 58MVP, 58UVB and most Carrier 58-series furnaces
Likely causes, most common first
- Hot and neutral swapped at a junction box, outlet, or the furnace itself
- On twinned furnaces, incorrect low-voltage (24 V) transformer phasing
When to call a technician
Reversed line-voltage polarity is an electrical wiring fault. A technician (or electrician) has to find and correct the crossed hot and neutral - this is not a homeowner repair.
A Carrier furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Sources
- 58MCB 4-Way Multipoise Fixed-Capacity Condensing Gas Furnace - Service Manual (Carrier, 58MCB single-stage condensing gas furnace) · checked 2026-08-25
- Troubleshooting Guide - Variable Speed, Two-Stage Gas Furnaces (Form 58-06T) (Carrier, 58MVP, 58UVB and other Carrier variable-speed two-stage gas furnaces (replaces 58MVP-10SM)) · checked 2026-08-25