C6
Bosch Fan Speed Feedback Fault

Boiler indicates fan speed signal error; may lock out with no ignition.

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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

Faulty Hall sensor or tachometer in fan, Broken fan feedback wire, Loose fan connector, Faulty PCB fan speed input

How to Fix / Troubleshooting

Safety: Isolate electrical supply and gas. Only remove the combustion cover if you are competent and local regulations allow.

Steps:

  • Inspect fan connector: Locate the multi‑pin plug on the fan. Ensure it is fully seated and free of corrosion or water damage.
  • Check wiring harness: Trace the harness back to the PCB. Look for crushed or abraded sections, especially where it passes through grommets.
  • Test continuity: With power off, use a multimeter to check continuity of the feedback wires between fan and PCB. Repair or replace the harness if open.
  • Observe start‑up: With covers refitted, restore power and call for heat. If the fan does not start or starts then stops quickly before ignition, the internal fan electronics may be faulty.

Professional repair: Replace the fan assembly if feedback is not being generated. PCB diagnosis should be done by a Bosch service engineer.

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Repair Difficulty

Hard 4/5

Required Part

Combustion Fan Assembly
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