A9
Bosch
DHW (Domestic Hot Water) Temperature Sensor Fault
Furnace stops and displays A9; may run briefly then shut down on temperature error
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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 DHW NTC sensor, Loose or corroded sensor connector, Shorted sensor cable, Control board DHW input failure
How to Fix / Troubleshooting
Safety: Turn off power and close water isolation valves if you need to remove any wet components.
- Locate DHW sensor: On combi-type Bosch furnaces/boilers, the DHW sensor is usually on the plate heat exchanger or DHW outlet pipe.
- Check connector: Unplug and replug the sensor connector. Clean any corrosion with electrical contact cleaner and allow to dry.
- Measure resistance: With the sensor disconnected, measure resistance and compare to Bosch NTC tables. Replace if open, shorted, or far from expected value.
- Inspect cable routing: Ensure the cable is not touching hot flue parts or sharp edges. Reroute or protect as needed.
If A9 remains after sensor replacement, the main control board may need replacement by a Bosch-authorized technician.
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Repair Difficulty
Medium
3/5
Required Part
DHW Temperature Sensor (NTC)
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