Code 21: Gas heating lockout — will NOT auto-reset
Safety stop — furnace off, call a technician
The manual’s check list is short and serious: mis-wired gas valve, or a defective control (valve relay). No homeowner step here.
From the table: Two-digit status codes — short flashes, then long flashes — source: Carrier 58YAV Service section - status code table (read the document, fetched 2026-10-30). Covers: Carrier 58-series and their Bryant/Payne twins. First digit = number of short flashes; second digit = number of long flashes. Code 33 is 3 short, pause, 3 long.
Carrier's manual gives code 21 the shortest entry in the lockout family: "Gas heating lockout — control will NOT auto reset. Check for: mis-wired gas valve; defective control (valve relay)." A control that suspects its own gas-valve relay, or a valve wired wrong, does not get to change its mind in three hours. A human has to look.
Context clue worth having: 21 often appears right after other work — a board replacement, a gas valve swap, a conversion — anywhere valve wiring was touched. If your furnace was serviced recently and now shows 21, tell the returning technician exactly that, and tell them the code before they arrive.
Safety stop — furnace off, call a technician — the safe list:
- Turn the furnace off at its power switch and leave it off.
- This code means the control thinks flame or gas is present when it should not be, or that a flame-safety switch has opened. Resetting it is not a fix.
- Call a licensed HVAC technician. If you smell gas at any point: leave the house and call your gas utility or 911 from outside.
Your whole job with a 21
Photograph the code. Turn the furnace off. Make the call. There is no homeowner checklist for this page because Carrier did not write one, and inventing one would be exactly the kind of confident nonsense this library exists to replace.
The two rules that outrank everything on this page.
1. Smell gas? Stop reading. Leave the house and call your gas utility or 911 from outside. No code is worth diagnosing through a gas smell.
2. Never bypass a safety switch — no jumping a pressure switch, no taping a limit, no propping a rollout. Those switches are the only thing between a furnace fault and a fire or carbon-monoxide problem. This library explains codes; it does not teach repairs inside the cabinet.
One honesty note before the tables: codes change between control boards, sometimes inside the same brand and the same decade. Every table below names the board and the manual it was transcribed from. The chart glued inside your own blower door is the final word for your furnace — if it disagrees with this page, believe the door.