E09
A.O. Smith
Exhaust Temperature Sensor / Flue Sensor Fault
A.O. Smith water heater shows E09 and will not operate
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Failed flue gas temperature sensor, Loose or damaged sensor wiring, Blocked or undersized venting causing high flue temps, Control board sensor circuit failure
How to Fix / Troubleshooting
Safety: Turn off power and gas. Do not operate the heater with suspected venting issues.
- 1. Inspect venting: Verify that the exhaust vent is installed per A.O. Smith specifications (diameter, length, number of elbows). Check for obstructions, sags, or disconnections.
- 2. Check flue sensor: Locate the flue gas temperature sensor on the exhaust outlet or flue collector. Ensure it is firmly mounted and the wiring harness is fully seated.
- 3. Wiring inspection: Look for melted insulation or discoloration near the flue sensor, which indicates overheating. Replace damaged wiring and sensor.
- 4. Replace sensor: If the sensor is open/shorted or physically damaged, install a new OEM flue temperature sensor.
- 5. Board evaluation: If a new sensor does not clear E09, the control board may have a failed input and should be tested and replaced by a professional.
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Repair Difficulty
Hard
4/5
Required Part
Flue Temperature Sensor
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