Bryant 315AAV Error Code 21: Gas Heating Lockout
What Does Code 21 Mean?
The Bryant 315AAV latches on a gas heating lockout when it reports code 21, and it is one of the few codes on this control that does not reset itself over time. The control has determined that the gas-heating circuit is not behaving as commanded and shuts it down permanently until power is cycled, precisely because the fault involves the gas valve or the relay that drives it.
Bryant's service data lists two causes to check: a mis-wired gas valve and a defective control (the valve relay on the board). In both cases the control cannot trust that it has proper command over the gas valve, so a latching lockout is the safe response. This is distinct from status code 22 (abnormal flame-proving signal), where the board sees flame with the valve de-energized, and from code 45 (control circuitry lockout), which flags an internal relay or software failure — but all three converge on the same theme: the control's authority over the gas valve is in question.
Because the required checks are gas valve wiring and control-board diagnosis, this is strictly a technician repair. A homeowner should not open or rewire the gas valve or its harness under any circumstances.
What You'll Notice
- The furnace will not produce heat and does not recover on its own, even after hours
- The status LED flashes two short then one long flash
- The lockout persists until power is cycled at the breaker or furnace switch
- The problem often follows recent service, a board replacement, or a gas valve replacement where wiring may 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 confirms the lockout does not clear on its own, then inspects the gas valve wiring against the wiring diagram for mis-landed or reversed connections, since a mis-wired valve is a listed cause and common right after service. If the wiring is correct, they evaluate the control board's valve relay for a defect. Because the fault latches, they also verify no unsafe command path exists before returning the furnace to service.
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 stays locked out and only a power cycle temporarily clears it
- The code appeared right after a gas valve, control board, or wiring repair
- Code 21 returns after cycling power
- Any situation involving the gas valve or its wiring — this is never a homeowner repair
Frequently Asked Questions
Why won't my Bryant 315AAV reset itself after code 21?
Code 21 is a latching gas heating lockout by design. Because it involves the gas valve or its relay, the control will not auto-reset; power must be cycled, and the wiring or board must be corrected first.
Can I just cycle the power and keep using the furnace?
Cycling power may clear the lockout, but the mis-wired valve or defective relay is still present and the code will return. It needs a technician to diagnose the gas valve wiring or control board.
Is code 21 dangerous?
It reflects a loss of trusted control over the gas valve, which is why the furnace latches off. Treat it seriously and have a qualified HVAC technician diagnose it before running the furnace.
Sources
✓ Verified against manufacturer service manual — March 2026