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Carrier Code 21 — Gas Heating Lockout

Code 21 (2 short + 1 long flashes)

Gas heating lockout — won't auto-reset, requires power cycle.

Brand: Carrier (Also covers Bryant, Payne)  |  Equipment: Gas Furnace

SAFETY: Code 21 is a non-resetting lockout for a reason — there's a real fault preventing safe gas heating. Don't attempt DIY gas valve work; call a licensed HVAC technician.

What This Code Means

Code 21 on Carrier furnaces means the control has locked out gas heating. Unlike soft lockouts, this one does not auto-reset after a timeout. Usually indicates a wiring issue at the gas valve or a defective control board.

Likely Causes (Ranked)

Common

Miswired gas valve

What: Incorrect wiring at the gas valve causes the control to detect a fault.

How to check: Inspect gas valve wiring against the wiring diagram on the furnace door.

Fix: Correct wiring per diagram; call a tech if unsure.

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Common

Defective control board

What: Internal failure on the control board triggers the lockout.

How to check: HVAC technician diagnostic; visible damage or burn marks on board.

Fix: Replace control board.

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Less common

Faulty gas valve

What: Internal gas valve fault triggers safety lockout.

How to check: Tech tests valve coil resistance and gas pressure.

Fix: Replace gas valve.

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Diagnostic Steps

  1. Cut power 60s — note if lockout returns
  2. Inspect gas valve wiring visually
  3. Check for any obvious damage to control board
  4. If reset doesn't work, call HVAC tech

Frequently Asked Questions

Will Carrier code 21 reset on its own?
No. Unlike soft lockouts, code 21 will not auto-reset after a timeout. You must power cycle and fix the underlying issue.
Can I fix code 21 myself?
Visual wiring check is OK. Gas valve work and control board diagnostics should be done by a licensed HVAC technician.
How much to fix Carrier code 21?
Wiring fix: free. Control board: $200-450. Gas valve: $150-300 plus labor.

Related Carrier Codes

Carrier Error Code 22Carrier Error Code 24

Related Symptoms

Furnace Not IgnitingFurnace Clicking Not Igniting

Need More Help?

Try our interactive symptom diagnostic tool or browse Carrier parts.

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