E9
Bosch
Exhaust / Flue Gas Sensor Failure
Bosch tankless water heater shows E9 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
Open or shorted flue gas temperature sensor, Damaged sensor wiring harness, Sensor dislodged from mounting point, Faulty control board input
How to Fix / Troubleshooting
Safety first: Turn OFF power and gas. Do not bypass safety sensors.
- Locate flue gas sensor: The sensor is typically clipped to the exhaust outlet or mounted on the heat exchanger near the flue collar. Verify it is firmly attached and not hanging loose.
- Inspect wiring: Follow the sensor wires back to the control board. Look for cuts, burns, or loose connectors. Reseat the connector at the board.
- Check resistance: With power off, disconnect the sensor and measure its resistance with a multimeter. Compare to the Bosch service chart for your model. Replace the sensor if it is open, shorted, or out of range.
- Control board check: If a known-good sensor still results in E9, the control board input circuit may be defective and should be replaced by a technician.
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Repair Difficulty
Medium
3/5
Required Part
Flue Gas Temperature Sensor
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