A8
Bosch
Return Temperature Sensor Failure
Furnace locks out with A8; unit does not fire and may show overheat or sensor 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
Defective return temperature sensor, Wiring harness damage, Sensor dislodged from pipe or well, Control board sensor circuit fault
How to Fix / Troubleshooting
Safety: Isolate electrical power and allow the system to cool.
- Find the return sensor: It is typically mounted on the return pipe entering the heat exchanger or in a sensor well.
- Inspect wiring: Follow the cable from the sensor to the control board. Look for cuts, melted insulation, or loose connectors. Repair or replace damaged wiring.
- Resistance check: Remove the sensor connector and measure resistance. Compare to Bosch specifications for the given water temperature. Replace if out of range.
- Mounting check: Ensure the sensor is firmly attached to the pipe or fully inserted into its pocket with thermal paste if specified.
Persistent A8 after sensor replacement indicates a likely control board issue that requires professional service.
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Repair Difficulty
Medium
3/5
Required Part
Return Temperature Sensor (NTC)
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