Furnace attempts to start but shuts down quickly; control shows A3 flame supervision error

Safety: Disconnect power and close the gas valve before touching internal components.

• Check electrode position: Remove the burner cover and compare the ionization electrode position to the Bosch installation diagram. The tip must be in the flame envelope. Adjust carefully and tighten mounting screws.
• Clean electrode and burner: Lightly sand the electrode rod and clean burner surfaces to ensure a stable flame. Remove rust flakes and dust with a soft brush and vacuum.
• Verify grounding: Ensure the burner assembly and control board ground wire are firmly attached to clean metal. Tighten loose screws and remove paint or corrosion under ground lugs.
• Observe flame: With panels in place and gas on, restore power and observe through the sight glass. The flame should be steady and blue. If it lifts off or goes out, gas pressure or combustion air may be incorrect and requires a technician.

If A3 continues, the control board or flame sensing circuit may be defective and should be tested and replaced by a qualified Bosch service technician.