HVACopediaBosch · Boiler
F0 Internal fault
What the control is telling you
The control reports an internal fault.
Applies to: Bosch Greenstar ZBR 21/28/35/42-3A boilers and ZWB 28/35/42-3A combi boilers
Likely causes
- Loose plug-in contact or ignition lead
- PCB fault
- Gas/air ratio off
Test and repair steps
- Warning: Disconnect mains power and close the gas cock before opening the appliance or working on the gas train.
- Hold reset for 3 seconds, then release to restart.
- Check plug-in contacts and ignition leads; check the gas/air ratio.
- Replace the PCB if the fault returns.
Models that show this code
4 models in Bosch Greenstar
Other codes on the same units
- d5 External supply temperature sensor fault (low-loss header)
- E2 Supply temperature sensor fault
- E3 Additional supply temperature sensor fault
- E9 Safety high limit or flue gas limiter tripped
- F1 Internal data error
- F7 Flame detected with burner off
- Ad DHW tank temperature not detected
- CC Outdoor temperature sensor not recognised
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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.