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 Carrier 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 senses the phase relationship of the incoming hot and neutral relative to ground. When that relationship is wrong, the board locks out heating to protect its electronics and to ensure the flame-sensing circuit reads correctly. 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 a similar rapid-flash indication.
Correcting this requires verifying which conductor is actually hot with a voltage meter and swapping the wires at the furnace disconnect or junction box. This is line-voltage work that must be performed with power off by a qualified electrician or HVAC technician — unlike code-off, the board here has power but refuses to run because the wiring is unsafe.
What You'll Notice
- The control board LED flashes rapidly and continuously, with no short/long pattern and no pause between flashes
- The furnace will not begin a heating sequence even though the thermostat is calling for heat
- The problem often appears right after new wiring, a receptacle or outlet change, a generator hookup, or work at the electrical panel
- On a twinned pair, one or both furnaces may show the rapid flash after the twinning kit was connected
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Hot and neutral wires reversed at furnace connection | Most common | ✗ Call a pro → |
How This Is Diagnosed
A technician confirms reversed polarity with a voltage meter at the furnace junction box, checking that the hot conductor lands on the line terminal and neutral on neutral, referenced to a verified ground. Because the board senses phase relative to ground, a swapped hot/neutral, a bootleg ground, or a missing ground can all read as reversed polarity, so the ground connection is checked as well. On twinned units the twinning harness is verified before the building wiring is disturbed.
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 rapid flash remains after confirming the furnace switch and breaker are on
- The problem started immediately after electrical work, a receptacle change, or a generator or transfer-switch hookup
- Line-voltage conductors need to be identified and swapped (this must be done with power off)
- The furnace is twinned and the twinning kit wiring needs to be verified
Frequently Asked Questions
Can reversed polarity hurt my Bryant 340MAV?
It stops the furnace from running as a safety measure and does not usually damage the board. It does mean the wiring is unsafe and should be corrected before the furnace is used.
I just had an outlet or panel worked on and now the LED flashes fast — is that related?
Very likely. Reversed polarity most often appears right after electrical work where the hot and neutral were swapped. Have whoever did the work recheck the connections.
✓ Verified against manufacturer service manual — March 2026