Bosch
No Power to Thermostat
Thermostat display is completely blank and does not power on.
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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
Tripped breaker to Bosch boiler/furnace/air handler, Blown low‑voltage fuse on control board, Disconnected R or C wire at thermostat or control board, Failed 24 V transformer in Bosch equipment
How to Fix / Troubleshooting
Safety: Turn off power at the main breaker panel before opening any HVAC equipment panels.
- Check breakers: Verify that the breaker for the furnace/air handler/boiler is ON and has not tripped. Reset if necessary.
- Inspect equipment fuse: On many control boards there is a 3–5 A blade fuse protecting the 24 V circuit. With power off, visually inspect and replace if blown, but only after checking for shorts in thermostat wiring.
- Check thermostat wiring: Remove the thermostat and confirm R and C wires are connected and not loose or corroded.
- Measure 24 V: With power on and using a multimeter, measure between R and C at the control board. If no voltage is present, the transformer or control board may be faulty and should be checked by a technician.
- Call a professional if unsure: Diagnosing and replacing transformers or control boards is best left to qualified service personnel.
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Repair Difficulty
Hard
4/5
Required Part
24 V Control Transformer or Bosch Control Board Fuse
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