Furnace will not start; control shows A2 lockout code after failed ignition attempts

Safety first: Turn off electrical power to the furnace at the breaker and close the gas shutoff valve before opening any panels. Allow the unit to cool fully.

Step-by-step checks:

• Reset lockout: Restore power and follow the Bosch control instructions to reset the A2 lockout (usually by pressing and holding the reset button on the burner control for several seconds). If the code returns, continue.
• Inspect flame/ionization electrode: Remove the burner access panel. Locate the ionization electrode near the burner. Check for soot, corrosion, or cracked porcelain. Clean the metal rod gently with fine emery cloth and wipe with a clean dry cloth. Reinstall with correct gap and position per Bosch service manual.
• Check ignition device: If your model uses a separate ignition electrode or hot-surface igniter, inspect for cracks or damage. Measure resistance with a multimeter (power off). Replace if open or out of spec.
• Verify gas supply: Confirm the manual gas shutoff valve is fully open. If other gas appliances are not working, contact your gas supplier. Do not attempt to adjust gas pressure yourself.
• Burner and air path: Check burner ports for heavy rust or debris. Vacuum loose dust from the burner area and ensure combustion air openings are not blocked.

When to call a professional: If A2 persists after cleaning the electrode and confirming gas supply, a licensed technician should test the gas valve, combustion settings, and burner control module.