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. Unlike most lockouts on this furnace, code 21 does not auto-reset — the furnace remains 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 actuator cannot close when commanded, which is a safety concern because gas can flow when it should not.
Because code 21 covers both a wiring/component fault and a potentially stuck-open valve, a technician must determine which cause is present before proceeding. This is distinct from code 22 (abnormal flame-proving signal), where a leaky valve lets gas pass and a flame is actually detected with the valve off; code 21 is about the valve control circuit and lockout logic itself.
What You'll Notice
- The furnace is completely locked out and will not restart on its own, even after waiting several hours
- No heat is produced, and cycling the thermostat does nothing until power is reset at the furnace
- The code returns immediately, or the furnace stays dead after a power reset, if the fault is still present
- In a stuck-open-valve case the inducer may keep running to purge even with no call for heat
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 → |
How This Is Diagnosed
Because code 21 covers both a wiring/relay fault and a possibly stuck-open valve, a technician first confirms the gas valve wiring against the diagram, then measures 24VAC at the gas valve terminals through the ignition sequence to see whether the control is commanding the valve correctly. If the board energizes the valve when it should not, the control (valve relay) is suspect; if the valve stays open with no command, the valve itself is replaced. Code 21 will not auto-reset, so power must be cycled after the repair.
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:
- Code 21 remains after cycling power (it will not auto-reset by design)
- The inducer keeps running even with no call for heat, which can indicate a stuck-open valve
- The furnace was recently serviced near the gas valve or control wiring
- Do not bypass or jumper the lockout to force the furnace to run
Frequently Asked Questions
Why won't my Bryant 340MAV reset after code 21?
Code 21 is designed not to auto-reset because it can involve a stuck-open gas valve, which is a safety concern. It stays locked out until the fault is repaired and power is cycled by a technician.
Is code 21 dangerous?
It can be. One of its causes is a gas valve that will not close on command, so the furnace intentionally locks out. Do not bypass it — have it diagnosed before running the furnace.
✓ Verified against manufacturer service manual — March 2026