Carrier Error Code 21: Gas heating lockout (Furnace)

Updated October 13, 2026 · by Aurora Star Tech Inc.

Carrier error code 21 on a furnace means: Gas heating lockout. It is also called Carrier diagnostic code 21.

Critical severityFurnace59TP6A · 58SB0A/58SB1A

Homeowner? What Carrier code 21 means for you

Stop and call Your furnace is reporting: Gas heating lockout. This code is a safety shutdown. Turn the furnace off and call a licensed technician. Do not keep resetting it. If you smell gas, leave the house and call your gas utility or 911 from outside.

Safe things you can check yourself

Check the thermostat: right mode (Heat or Cool), set a few degrees past room temperature, and fresh batteries if it uses them.
Make sure the unit has power: its on/off switch (often looks like a light switch near the unit) is ON and the breaker in the electrical panel is not tripped.

Call (647) 657-2977 Book a service call

Licensed technicians serving Vaughan, Toronto and the GTA.

What does Carrier error code 21 mean?

The control locked out of gas heating; it points to the gas valve relay or gas valve wiring.

Applies to: 59TP6A two-stage variable-speed ECM multipoise condensing gas furnace, all sizes (040-10 to 120-22)

Common causes of Carrier code 21

Gas valve relay on control stuck closed
Miswire or short on the gas valve wire

Safety: Gas, electrical and refrigerant work must be done by a licensed technician. Turn off power and gas before servicing, never bypass a safety control, and always follow the manufacturer's manual for your exact model.

Carrier models that use code 21

Show all 22 models

58SB0A045E14--12 58SB0A045E17--12 58SB0A070E14--12 58SB0A070E17--12 58SB0A070E17--16 58SB0A070E21--16 58SB0A090E17--14 58SB0A090E21--16 58SB0A090E21--20 58SB0A090E24--20 58SB0A110E21--20 58SB0A110E24--20 58SB0A135E24--20 58SB0A155E24--20 59TP6A040-10 59TP6A040-12 59TP6A060-12 59TP6A060-14 59TP6A080-16 59TP6A080-20 59TP6A100-20 59TP6A120-22

Other Carrier Furnace codes

Code	Meaning
13.1	Limit lockout - main limit open over 3 minutes
14	Ignition lockout
14.1	Ignition lockout after 4 tries
14.2	Lockout - flame lost 3 times after 70 s of heating
14.3	Lockout - 7 flame-loss events in one heat call
15	Blower motor lockout
21.1	24 VAC on gas valve when it should be off
22	Abnormal flame-proving signal
22.1	False flame
23	Pressure switch did not open
23.1	Main pressure switch stuck closed
23.2	Secondary pressure switch stuck closed

All Carrier Furnace error codes →

Carrier code 21: homeowner questions

What does Carrier code 21 mean?

Gas heating lockout. The control locked out of gas heating; it points to the gas valve relay or gas valve wiring.

Is it safe to keep using my furnace with code 21?

No. Code 21 is a safety shutdown, so keep the furnace off until a licensed technician has checked it.

Can I reset Carrier code 21 myself?

Do not keep resetting it. Repeated resets can hide a safety problem; a licensed technician needs to find the cause.

Who can fix Carrier code 21 near me?

Aurora Star Tech Inc. repairs Carrier furnaces in Vaughan, Toronto and across the GTA. Call (647) 657-2977 or book a service call online.

Homeowner help: Furnace Not Turning On: What to Check · Furnace Blinking Light: What the Flash Codes Mean

Troubleshooting guides

Blowing cold air All brands · Furnace
No heat All brands · Furnace
Short cycling All brands · Furnace
Burners will not light or stay lit - Carrier 58SB0A/58SB1A Carrier · Furnace
Furnace not responding to thermostat - Carrier 58SB0A/58SB1A Carrier · Furnace

Source: Carrier 59TP6A Installation, Start-up, Operating and Service Instructions, Fig. 66 Troubleshooting Guide (cont.), p.89. Written in our own words as a summary; always check the original manual.

General guidance for qualified technicians. The manufacturer's installation and service manual and local codes always take priority. Aurora Star Tech is not affiliated with, endorsed by or sponsored by any equipment manufacturer; brand and model names are used only to identify equipment and belong to their owners.