Bryant 315AAV Error Code 21: Gas Heating Lockout
What Does Code 21 Mean?
Status code 21 on the Bryant 315AAV is a gas heating lockout. Unlike most fault codes on this furnace, code 21 does not auto-reset — the furnace will remain locked out until the fault is repaired and power is cycled.
The service documentation identifies two causes: a miswired gas valve and a defective control board gas valve relay. A miswired valve means the 24VAC signal wires connecting the control board to the gas valve are incorrectly connected, so the valve cannot receive the proper open or close command. A defective valve relay on the control board means the internal switching component has failed and cannot reliably control the gas valve.
Diagnosis requires checking for 24VAC at the gas valve terminals during the ignition sequence with a voltmeter, and verifying that the wiring matches the diagram in the installation manual. Because live-circuit testing near a gas valve is involved, the repair must be performed by a licensed HVAC technician.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Mis-wired gas valve | Common | ✗ Call a pro → |
| Defective control board (valve relay) | Common | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- Contact a technician — code 21 will not auto-reset and the furnace will not heat until the gas valve wiring or control board is repaired
- Do not attempt to bypass the gas heating lockout; it is a safety protection against a gas valve circuit fault
✓ Verified against manufacturer service manual — March 2026