A.O. Smith tankless or high-efficiency heater shows E08 and stops heating

Safety: Turn off power and gas. Allow the unit to cool completely before servicing.

• 1. Check water flow: Open multiple hot water taps and observe flow. If flow is weak, clean the cold water inlet filter screen and any external filters or strainers.
• 2. Inspect recirculation pump: On models with a built-in or external recirc pump, verify the pump is running when there is a call for hot water. Listen for operation and feel for vibration. Replace or service the pump if it is seized or silent.
• 3. Descale heat exchanger: Connect a pump and hoses to the service valves and circulate a descaling solution (such as white vinegar or manufacturer-approved cleaner) through the heat exchanger for the recommended time. Flush thoroughly with clean water afterward.
• 4. Verify temperature settings: Ensure the setpoint is not excessively high. Lowering the set temperature can reduce overheat conditions.
• 5. Sensor check: If overheating persists, test or replace the outlet temperature sensor and high-limit sensor.

If E08 recurs after descaling and restoring proper flow, contact a technician to evaluate the control board and safety circuits.