Bryant 355MAV Error Code 21: Gas Heating Lockout
What Does Code 21 Mean?
Status code 21 on the Bryant 355MAV indicates a gas heating lockout. Unlike most other lockout codes on this furnace, code 21 will NOT automatically reset — the furnace will remain locked out until power is interrupted after the problem is repaired. This permanent lockout exists because the conditions that trigger it could allow gas to flow unsafely.
The two primary causes are a mis-wired gas valve and a defective control board valve relay. A mis-wired gas valve means the electrical connections between the control board and the gas valve are incorrect, which could cause the valve to open at the wrong time or fail to close properly. A defective valve relay on the control board means the internal switch that controls power to the gas valve is malfunctioning.
Both of these conditions are serious because they involve the system that controls gas flow to the burners. An improperly controlled gas valve could release unburned gas into the furnace and your home. Only a qualified HVAC technician should diagnose and repair this issue. After the repair, the technician will need to cycle power to clear the lockout and verify proper operation.
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:
- Immediately — code 21 is a permanent lockout indicating a gas safety issue that requires professional repair
- The furnace will not reset on its own and power must be interrupted after the problem is fixed
✓ Verified against manufacturer service manual — March 2026