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Bosch · Boiler

F1 Internal data error

Medium severity Bosch Greenstar

What the control is telling you

A data error in the control memory.

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

Likely causes

  • Corrupted settings

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. Restore factory defaults with service function 8.E and re-enter the commissioning data.

Models that show this code

4 models in Bosch Greenstar

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

Other codes on the same units

  • E2 Supply temperature sensor fault
  • E3 Additional supply temperature sensor fault
  • E9 Safety high limit or flue gas limiter tripped
  • F0 Internal fault
  • F7 Flame detected with burner off
  • Ad DHW tank temperature not detected
  • CC Outdoor temperature sensor not recognised
  • EA No flame detected

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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.