Bryant 340MAV Error Code Rapid Flashing: Reversed Line Voltage Polarity
What Does Code Rapid Flashing Mean?
When the diagnostic LED on the Bryant 340MAV control board flashes rapidly without a pause, the integrated furnace control has detected that 115VAC line voltage polarity is reversed at the furnace. The hot and neutral conductors are swapped.
Reversed polarity prevents the furnace from operating because the control board's internal circuit relies on correct phase relationship between the incoming hot and neutral conductors. The board detects the reversed polarity and locks out heating to protect electronics. The service documentation notes that on twinned 340MAV installations (where two furnaces are paired together), the twinning kit instructions must also be consulted, as improper twinning connections can cause similar rapid-flash faults.
Correcting this requires verifying which conductor is hot with a voltage meter and swapping the wires at the furnace disconnect box or junction box. This work must be performed with power off by a qualified electrician or HVAC technician.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Hot and neutral wires reversed at furnace connection | Most common | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- Call a licensed electrician or HVAC technician to correct the reversed polarity — line voltage work must be done safely with power off
- For twinned installations, consult the twinning kit instructions in addition to correcting line voltage polarity
✓ Verified against manufacturer service manual — March 2026