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Bosch · Boiler

C6 Fan not running

High severity Bosch Greenstar

What the control is telling you

The control does not get the expected fan response.

Applies to: Bosch Greenstar ZBR 21/28/35/42-3A boilers and ZWB 28/35/42-3A combi boilers

Likely causes

  • Fan wiring fault
  • Failed fan

Test and repair steps

  1. Warning: Disconnect mains power and close the gas cock before opening the appliance or working on the gas train.
  2. Check the fan connectors and leads.
  3. Replace the fan if wiring is good.

Models that show this code

4 models in Bosch Greenstar

ZBR21-3AZBR28-3AZBR35-3AZBR42-3A

Other codes on the same units

  • A8 Communication fault
  • b1 Code plug not detected
  • b2/b3/b4/b5/b6 Internal data error
  • b7 Burner control fault
  • d3 High limit / temperature limiter circuit open
  • d5 External supply temperature sensor fault (low-loss header)
  • E2 Supply temperature sensor fault
  • E3 Additional supply temperature sensor fault

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Source: Bosch Greenstar ZBR/ZWB 21-42-3A service manual 6721853955 (2025-11), Table 38 Faults shown on the display, p.74-75; Greenstar combi 100 p/151 p service manual 6721853953 (2025-11), Table 33. Summarized in our own words; check the original manual for your model.

Homeowner looking at this code? See what it means for you and when to call a technician.

For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.