Carrier furnace code 21: Gas Heating Lockout (No Auto-Reset)
Professional repair
What does code 21 mean on a Carrier furnace? Official: mis-wired gas valve, a stuck-closed gas-valve relay on the control, or a shorted gas-valve wire. This one does NOT auto-reset.
This condition involves the gas or combustion side. If you smell gas or a CO alarm sounds: leave, then call your gas utility. Diagnostic info here is reference — combustion repairs are for licensed technicians. All six emergency signs →
Gas-side control fault — licensed tech only. DIY part cost: $120–250 — a pro visit for this class of fault typically runs $150–350.
What causes it (in order of likelihood)
- Control board gas-valve relay stuck TECH
- Gas-valve wiring error/short TECH
Parts referenced by this code
Always match parts against your model's parts list — part numbers vary within a series.
Questions about this code
What does code 21 mean on a Carrier furnace?
Official: mis-wired gas valve, a stuck-closed gas-valve relay on the control, or a shorted gas-valve wire. This one does NOT auto-reset.
How do I fix Carrier furnace code 21?
Causes include a miswired gas valve, a stuck gas-valve relay on the control, or a shorted valve wire. Stop troubleshooting; this needs a licensed technician. If you smell gas, leave and call from outside.
How much does it cost to fix Carrier furnace code 21?
Cost depends on whether the problem is wiring or the control's gas-valve relay. A licensed technician's diagnosis is needed before any price is meaningful.
Applies to
58STA · 58STX
Different Carrier generations use different code maps. If your series isn't listed, don't assume this meaning — check your blower-door label.
Same control platform, same code: Bryant furnace code 21 · Payne furnace code 21
Verified against Carrier 58STA/58STX Installation & Service Instructions (1106202L), p.37/48
Last verified 2026-07-06 · page updated 2026-10-03 · how we verify
Related Carrier furnace codes
These codes use the same status led — short + long flash codes system.