CARRIER DIAGNOSTIC REFERENCE STATUS LED — SHORT + LONG FLASH CODES DOC CARRIER-21

Carrier furnace: Code 21 — Gas Heating Lockout (No Auto-Reset)

Q: What does code 21 mean on a Carrier furnace?

Code 21 — Gas Heating Lockout (No Auto-Reset). 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.

YOUR LIGHT: 2 SHORT + 1 LONG FLASHES — MATCH IT ABOVE

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.

Gas / CO warning

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.

See what causes this code ↓ Get repair quotes near you

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

HK42FZ034 — Furnace Control Board

$120–250 · also sold as HK42FZ013

Amazon HVACDirect

Always match parts against your model's parts list — part numbers vary within a series.

Applies to

58STA · 58STX · Bryant 310/311/312 series · Payne PG8M series

Different Carrier generations use different code maps. If your series isn't listed, don't assume this meaning — check your blower-door label.

Verified against Carrier 58STA/58STX Installation & Service Instructions (1106202L), p.37/48

Last verified 2026-07-06 · how we verify