Carrier Furnace Code 45
Soft lockoutShown as: 4 short flashes, 5 long flashes
Control circuitry lockout. The board detected an internal failure and locks out, auto-resetting after 1 hour.
Manual wording: “45 CONTROL CIRCUITRY LOCKOUT - Auto-reset after 1 hour lockout due to: - Flame sense circuit failure. - Gas valve relay stuck open. - Software check error.”
Documented for: 58SC0A, 58MCB, 58MVP, 58UVB
Likely causes, most common first
- Flame sense circuit failure
- Gas valve relay stuck open
- Software check error
What you can safely check yourself
- Turn the furnace off at the service switch for a minute and back on once - the manual says resetting power clears this lockout
When to call a technician
If code 45 comes back after a power reset, the manual's own instruction is to replace the control board - that is a technician job.
A Carrier furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Sources
- 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
- Installation, Start-Up, Operating and Service and Maintenance Instructions - 58SC0A/58SC1A (Carrier, 58SC0A/58SC1A single-stage 4-way multipoise condensing gas furnace) · checked 2026-08-25
- 58MCB 4-Way Multipoise Fixed-Capacity Condensing Gas Furnace - Service Manual (Carrier, 58MCB single-stage condensing gas furnace) · checked 2026-08-25