Bryant 355MAV Error Code 21: Gas Heating Lockout
What Does Code 21 Mean?
Status code 21 on the Bryant 355MAV is a gas heating lockout, and unlike most lockouts on this furnace it will NOT reset on its own. The furnace stays locked out until power is cycled after the underlying problem is repaired. This permanent behavior is deliberate, because the conditions that trigger code 21 involve the control of gas flow.
Bryant lists two things to check for this code: a mis-wired gas valve and a defective control (valve relay). A mis-wired valve means the connections between the board and the gas valve are wrong, so the valve could be energized at the wrong moment or fail to close as commanded. A defective valve relay means the switch on the control board that powers the gas valve is malfunctioning.
Both conditions are serious because they concern the system that admits gas to the burners. This is not the same as code 22 (Abnormal Flame-Proving Signal), where flame is detected with the valve de-energized; code 21 is specifically about the valve control circuit itself. Only a qualified HVAC technician should diagnose and repair it, then cycle power to clear the lockout and verify safe operation.
What You'll Notice
- The furnace produces no heat and does not attempt to restart on its own
- Cycling the thermostat does not clear the fault — it stays locked out
- The status LED flashes code 21 (two short flashes, one long flash)
- The condition often follows recent service, a board swap, or gas valve work where wiring could have been disturbed
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Mis-wired gas valve | Common | ✗ Call a pro → |
| Defective control board (valve relay) | Common | ✗ Call a pro → |
How This Is Diagnosed
A technician verifies the gas valve wiring against the board's wiring diagram, confirming each conductor lands where it should so the valve is only energized when commanded. The control's valve relay is then checked to confirm it opens and closes correctly rather than sticking.
Because the fault will not auto-reset, the technician clears it by cycling power only after the wiring or relay problem is corrected and safe valve operation is confirmed. Every step here touches the gas valve control circuit, so it is strictly professional work.
When to Call a Professional
This code involves components that are not homeowner-serviceable, so have a licensed HVAC technician diagnose and repair it. Keep in mind:
- The furnace shows code 21 and will not reset on its own
- The lockout appeared after recent service, a control-board replacement, or gas valve work
- You smell gas near the furnace at any time
- You need the lockout cleared, which should only be done after the wiring or relay is repaired
Frequently Asked Questions
Why won't my Bryant 355MAV reset after a code 21?
Code 21 is a permanent lockout by design because it involves the gas valve control circuit. It will not auto-reset; power must be cycled only after a technician repairs the mis-wired valve or defective relay.
Is code 21 dangerous?
It is treated as serious because it concerns how gas is admitted to the burners. Leave it to a qualified technician, shut off the gas if you smell any, and do not attempt to clear the lockout without fixing the cause.
Sources
✓ Verified against manufacturer service manual — March 2026