Bryant 340MAV Error Code 34: Ignition Proving Failure

What Does Code 34 Mean?

Status code 34 on the Bryant 340MAV indicates an ignition proving failure. The furnace either failed to detect a flame after the gas valve opened, or the flame was detected but then lost. The control will try three more times before escalating to lockout code 14.

The flame sensor is a thin metal rod positioned in the burner flame. It works by detecting a small electrical current (microamps) that flows through the flame. When the sensor rod becomes coated with oxidation, this current drops below the minimum threshold (0.5 microamps DC minimum, with 4.0-6.0 being the normal range), and the control board interprets this as no flame present — even if the burners are actually lit.

The most common cause is oxide buildup on the flame sensor rod. Other causes include a closed manual gas valve, a defective hot surface ignitor, low inlet gas pressure, inadequate flame carryover between burners, or a ground continuity issue with the control. The green/yellow wire must be connected to the furnace sheet metal for proper flame sensing. If the flame sensor is not grounded correctly, it will not read the flame signal.

Common Causes

How to Fix It: Clean the Flame Sensor

What You'll Need

• 1/4-inch hex head driver or nut driver
• a Scotch-Brite pad
• Flashlight

Steps

1. Turn off electrical power at the breaker and shut off the gas supply valve Locate the furnace circuit breaker and flip it to OFF. Turn the gas shutoff valve to the OFF position (perpendicular to the pipe). If you smell gas, leave immediately and call your gas company.
2. Locate the flame sensor Open the furnace access panel and look near the burner assembly. The flame sensor is a thin metal rod (usually bent at an angle) mounted with a porcelain insulator. It typically has a single wire connected to it and is held in place by one 1/4-inch screw.
3. Remove the flame sensor Disconnect the wire from the flame sensor. Remove the mounting screw and carefully pull the sensor out. Handle it by the porcelain insulator or mounting bracket, not the rod itself.
4. Clean the sensor rod Gently rub the metal rod with a Scotch-Brite pad until it is clean and shiny, removing all oxide buildup. Do not bend the rod or damage the porcelain insulator. Some HVAC professionals recommend using a Scotch-Brite pad as it leaves no abrasive residue.
5. Reinstall the flame sensor Place the sensor back in its bracket, secure the mounting screw, and reconnect the wire. Ensure the sensor rod will be positioned in the flame path when the burners ignite. Verify the green/yellow ground wire is connected to the furnace sheet metal — this is required for proper flame sensing.
6. Verify the gas supply is on Check that the manual gas shutoff valve near the furnace is open (handle parallel to the pipe).
7. Restore power and gas, then test Turn the gas supply valve to ON and flip the circuit breaker to ON. Set your thermostat to call for heat and watch the ignition sequence through the sight glass or access panel opening.

When to Call a Professional

Contact a licensed HVAC technician if:

• The burners still fail to stay lit after cleaning the flame sensor
• The hot surface igniter does not glow during the ignition sequence
• You can hear gas flowing but it does not ignite
• The flame appears yellow, lazy, or rolls out of the combustion chamber
• Code 34 occurs repeatedly even with a clean flame sensor

✓ Verified against manufacturer service manual — March 2026