Carrier and Bryant 21 Error Code: Gas heating lockout
Updated October 2026 · Sourced from Carrier and Bryant documentation
🛑 Turn it off and call a licensed pro
The control has locked out gas heating and will not auto reset. The label says to check for a mis-wired gas valve or a defective control valve relay.
Is it safe to keep running? No. Turn it off and get it checked. Turn the thermostat to Off and call a licensed technician. Gas valve wiring and the control board are professional work.
21 at a glance
| Code / how it shows | Meaning | Applies to |
|---|---|---|
| 21 Status LED flashes: first digit is short flashes, second digit is long flashes | Gas heating lockout | Carrier 59SC5A single-stage 4-way multipoise condensing gas furnace (Series 1 and 2) |
🛑 Stop and call a licensed pro if
- Turn the thermostat to Off and call a licensed technician. Gas valve wiring and the control board are professional work.
- You smell gas. Leave the house, do not touch switches, and call your gas utility from outside.
- Flame rollout, limit or pressure switch codes that come back after one reset.
- Anything involving the control board, gas valve, flame sensor, igniter or venting.
- A carbon monoxide alarm sounds. Get everyone outside and call 911 or the fire department.
Safe first steps for any Carrier and Bryant code
- Count the LED flashes through the blower door window before you switch anything off. Cutting power can clear the code.
- Replace or clean the furnace air filter.
- Check the thermostat: correct mode, set above room temperature, and fresh batteries.
- Make sure the furnace power switch and breaker are on. If the furnace has locked out, turning the power off and back on once restarts it.
- Make sure the gas valve is in the ON position, following the lighting instructions on the furnace label only.
- On high-efficiency furnaces, look at the outside vent and intake pipes for snow, ice, leaves or nests (look only, do not take pipes apart).
Source: Carrier 59SC5A Installation, Start-up, Operating and Service and Maintenance Instructions, Fig. 62 Service Label, p.67. Codes are series-specific, so your unit's own manual is the final word. Reviewed October 2026.
Shaun Cain · Certified HVAC & Refrigeration Technician · Ask an HVAC technician
Related Carrier and Bryant codes
LED off : Status LED off (no power to control)LED on : Status LED steady on (normal)Rapid flashing : Rapid flashing LED (reversed polarity)11 : No previous fault code stored12 : Blower on after power up13 : Limit circuit lockout14 : Ignition lockout22 : Abnormal flame-proving signal23 : Pressure switch did not open24 : Secondary voltage fuse is open31 : Pressure switch did not close or reopened33 : Limit circuit fault (limit or rollout switch open)