Carrier Furnace Code 21
Hard lockoutShown as: 2 short flashes, 1 long flash
Gas heating lockout. The control detected 24 VAC on the gas valve when it should not be there. This lockout does NOT auto-reset.
Manual wording: “21 GAS HEATING LOCKOUT - Control will NOT auto reset. Check for: - Mis-wired gas valve. - Defective control (valve relay).”
Documented for: 58SC0A, 58MCB, 58MVP, 58UVB
Likely causes, most common first
- Stuck-closed gas valve relay on the control board
- Mis-wired or shorted gas valve wire
When to call a technician
Code 21 means the control can no longer guarantee it can shut the gas valve off. Shut the furnace off at the service switch and call a technician - this one does not clear on its own.
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