Bryant 355MAV Furnace Error Code 34: Ignition Proving Failure

What Does Code 34 Mean?

Status code 34 on the Bryant 355MAV is an ignition proving failure. The furnace ran its ignition sequence — inducer started, igniter heated, gas valve opened — but the flame sensor did not detect a stable flame, or the flame was lost shortly after being established.

This is the active fault that precedes lockout. The control will try up to three more times before escalating to code 14 (Ignition Lockout) and shutting down for 3 hours. If the flame signal is lost during the blower on-delay period, the blower comes on for the selected blower off-delay to clear the heat exchanger.

The most common cause is a dirty flame sensor — oxide buildup on the rod that keeps it from generating enough microamp signal to confirm the flame. The manufacturer's manual lists cleaning the flame sensor for this fault, and it is a homeowner-safe task. A defective hot surface igniter, low inlet gas pressure, or a failed gas valve are less common and need a technician.

What You'll Notice

• The furnace lights or tries to light, then drops out after a few seconds
• You hear the inducer and igniter cycle repeatedly without steady heat
• The blower may run briefly after a failed attempt to clear residual heat
• The status LED flashes code 34 (three short flashes, four long flashes)
• If it keeps failing, the furnace escalates to a code 14 lockout

Common Causes

How This Is Diagnosed

Cleaning the flame sensor is both the first step and the most common fix, since a fouled rod is the usual cause. If cleaning does not resolve it, a technician measures the sensor's microamp signal against spec, checks the hot surface igniter for a proper glow and current draw, verifies inlet gas pressure, and confirms grounding and wiring at the sensor, igniter, and valve.

How to Fix It: Clean the Flame Sensor

What You'll Need

• Scotch-Brite pad
• 1/4-inch nut driver or screwdriver
• Flashlight

Steps

1. Turn off power at the furnace switch or breaker and shut off the gas supply Set the furnace disconnect to OFF (or trip its breaker) and close the manual gas shutoff valve before opening the burner compartment. If you smell gas, leave the house immediately and call your gas company from outside.
2. Remove the flame sensor Find the single sensor rod with a porcelain base near the burners and igniter. Remove its mounting screw, note the wire connection, and slide the rod out carefully.
3. Polish the sensor rod Gently clean the flame sensor rod until the metal is dull-bright. Bryant's guide lists fine steel wool as the cleaning material, but many HVAC technicians prefer a Scotch-Brite pad because it leaves no abrasive residue on the rod; if you do use steel wool, wipe the rod thoroughly afterward, since stray metal fragments left on the rod or porcelain can cause a short. Do not clean or scratch the white porcelain base, and take care not to bend the rod.
4. Reinstall and restore power Refit the rod, secure the screw, reconnect the wire, and close the panel. Turn the gas back on, restore power, and cycle the furnace switch if the related code 14 lockout is present.

When to Call a Professional

Contact a licensed HVAC technician if:

• Code 34 keeps appearing after the flame sensor has been cleaned
• The igniter does not glow red-hot during the sequence
• The flame lights briefly and then goes out within seconds
• You smell gas but the burners do not stay lit
• The code escalates to lockout code 14 repeatedly

✓ Verified against manufacturer service manual — March 2026