Display shows 'AO2' and heater shuts down; may indicate secondary air/vent fault.

Safety first: Turn off power and gas. Venting issues can be hazardous.

Steps to address AO2 error:

• Inspect entire vent system: Look for low spots where condensate can collect, sagging sections, or partial obstructions.
• Check condensate drains (if applicable): Ensure any condensate drain lines from the vent or heat exchanger are clear and properly pitched.
• Monitor blower performance: Listen for speed fluctuations or surging. Replace blower if it cannot maintain consistent airflow.
• Test air pressure switch: Use a manometer to verify that the switch closes at the specified pressure. Replace if it is slow or intermittent.

Note: AO2 faults that recur after basic cleaning should be evaluated by a professional with combustion test equipment.