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Bosch · Boiler

A8 Communication fault

Medium severity Bosch Greenstar

What the control is telling you

The boiler control has lost communication on the BUS wiring.

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

Likely causes

  • Damaged or loose BUS wiring
  • Faulty connected control

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. Inspect the BUS wiring between the boiler and the control; replace damaged wiring.
  3. Replace the connected control if the wiring is good.

Models that show this code

4 models in Bosch Greenstar

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

Other codes on the same units

  • A7 DHW temperature sensor fault
  • b1 Code plug not detected
  • b2/b3/b4/b5/b6 Internal data error
  • b7 Burner control fault
  • C6 Fan not running
  • d3 High limit / temperature limiter circuit open
  • d5 External supply temperature sensor fault (low-loss header)
  • E2 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.