HVACopediaBosch · Tankless water heater
BB Fan calibration after missed ignitions
What the control is telling you
The fan was recalibrated because of missed ignitions.
Applies to: Bosch Greentherm T9800 SE 160/199 and T9900 SE 160/199 / i 199 tankless water heaters
Likely causes
- Low gas inlet pressure
- Combustion out of adjustment
Test and repair steps
- Check gas inlet pressure.
- Run the gas and air tuning procedure.
Models that show this code
6 models in Bosch Greentherm T9800 SE, Bosch Greentherm T9900 SE
Other codes on the same units
- F8 Control unit internal error
- F9 Gas valve driver fault
- AA Flue gas temperature too high, power limited
- AD Power derate warning
- BC Repeated fan calibration with similar values
- CA Water flow above 9.2 gpm
- CE Flue blockage during operation (siphon pressure)
- CF Flue blockage during operation (insufficient air)
Save it for the next call
Star this code in the HVACopedia app and keep the whole library on your phone for jobs with no signal.
Source: Bosch Greentherm T9800 SE 160/199 installation 6720816815 (06.2023), Table 24 Error code diagnostics, p.65-69; Bosch Greentherm T9900 SE 160/199 | i 199 installation 6 720 813 635 (2024-06), section 9.1 Error code diagnostics, p.68-73. 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.