Bryant 340MAV Error Code 21: Gas Heating Lockout
What Does Code 21 Mean?
Status code 21 on the Bryant 340MAV is a gas heating lockout. 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 three causes: a miswired gas valve, a defective control board gas valve relay, and a stuck-open gas valve. A miswired valve means the 24VAC signal wires connecting the control board to the gas valve are incorrectly connected. A defective valve relay means the internal switching component on the control board has failed. A stuck-open gas valve means the valve's mechanical actuator cannot close when commanded, which is also a safety concern as gas may flow when it should not.
Because code 21 encompasses both a wiring/component fault and a potentially stuck-open valve, a technician must determine which cause is present before proceeding. Diagnosis involves checking 24VAC at the gas valve terminals during the ignition sequence and verifying the valve opens and closes correctly. A stuck-open gas valve must be replaced.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Mis-wired gas valve | Common | ✗ Call a pro → |
| Defective control board (valve relay) | Common | ✗ Call a pro → |
| Stuck-open gas valve | Common | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- Contact a technician immediately — code 21 will not auto-reset, and a stuck-open gas valve is a safety concern
- Do not attempt to bypass the gas heating lockout; the furnace will not heat safely until the cause is identified and repaired
✓ Verified against manufacturer service manual — March 2026