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Carrier 21: Gas heating lockout. Control will NOT auto-reset

Ref. CARRIER-21 · From the Carrier furnace manual · Updated 2026-10-23

Status code21Carrier
What it means

Gas heating lockout. Control will NOT auto-reset.

Call a profurnace
How it shows: Status LED: 2 short flash(es) then 1 long flash(es) (code 21)

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§1Likely causes

  • Mis-wired gas valve
  • Stuck closed gas valve relay on control (defective control)
  • Short to gas valve wire

§2Safe DIY steps

  1. Turn off power and wait 5 minutes to retry
  2. Techniciancheck gas valve wiring and the control valve relay
Safety firstIf you smell gas, do not touch any electrical switch or use a phone in the building; leave immediately and call your gas supplier from outside. Turn off electrical power at the furnace switch or breaker before opening any panel, and never bypass or jumper a limit, rollout or pressure switch. Gas valve, burner, venting, wiring and control-board work should be done by a licensed HVAC technician.

§3When to call a pro

Call a licensed HVAC technician: always; this lockout does not clear on its own and points to a gas valve circuit fault.

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§4Other Carrier codes

13Carrier

Limit circuit lockout: a limit or flame rollout switch was open longer than 3 minutes. Control auto-resets after 3 hours (see code 33).

14Carrier

Ignition lockout: system failed to ignite gas and prove flame in 4 attempts. Control auto-resets after 3 hours (see code 34).

22Carrier

Abnormal flame-proving signal: flame is proved while the gas valve is de-energized. Inducer runs until the fault is cleared.

23Carrier

Pressure switch did not open.

31Carrier

Low (LPS) or high (HPS) pressure switch did not close or reopened.

24Carrier

Secondary voltage (24 V) fuse is open.