HVACopediaBosch tankless water heater error codes
44 codes with causes and test steps.
| Code | Meaning | Severity |
|---|---|---|
| A0 | Water temperature sensors disconnected or damaged | Medium |
| A1 | Cabinet air temperature above 158 F (70 C) | Medium |
| A2 | External water temperature sensor / aquastat fault | Low |
| A3 | Flue gas temperature sensor fault | Medium |
| A4 | Air temperature sensor fault | Medium |
| A5 | Condensing unit flue temperature sensor fault | Medium |
| A6 | Inlet cold water temperature sensor fault | Medium |
| A7 | Heat exchanger hot water temperature sensor fault | Medium |
| A8 | Outlet hot water temperature sensor fault | Medium |
| A9 | Output power too low | Medium |
| C1 | Flue blockage at start-up | High |
| C2 | Flue blockage at start-up | High |
| C3 | Flow sensor sees no recirculation | Low |
| C5 | Bypass water valve fault | Medium |
| C7 | Fan fault | High |
| C8 | Main water valve fault | Medium |
| C9 | Barometric pressure sensor damaged | Medium |
| E0 | Control unit internal error | Medium |
| E1 | Hot water above 194 F (90 C) | High |
| E3 | Flue gas temperature too high, burner cut off | High |
| E4 | Cabinet air too hot, burner cut off | High |
| E5 | Condensing unit flue too hot, burner cut off | High |
| E7 | Control unit internal error | Medium |
| E8 | Ionization test error | Medium |
| E9 | Thermal fuse open | High |
| F2 | Control unit internal error | Medium |
| F3 | Control unit internal error / configuration | Medium |
| F7 | Control unit internal error | Medium |
| F8 | Control unit internal error | Medium |
| F9 | Gas valve driver fault | High |
| AA | Flue gas temperature too high, power limited | Medium |
| AD | Power derate warning | Low |
| BB | Fan calibration after missed ignitions | Medium |
| BC | Repeated fan calibration with similar values | Medium |
| CA | Water flow above 9.2 gpm | Low |
| CE | Flue blockage during operation (siphon pressure) | High |
| CF | Flue blockage during operation (insufficient air) | High |
| EA | Ignition failure | High |
| EB | Control unit internal error | Medium |
| EC | Flame lost during operation | High |
| EE | Gas valve modulation coil disconnected | High |
| EF | Wrong gas type configured | High |
| FA | Gas leak in the gas path | Critical |
| FC | Touch key held over 20 seconds | Low |
Models covered
Greentherm T9800 SE 160Greentherm T9800 SE 199Greentherm T9800 SEC 199Greentherm T9900 SE 160Greentherm T9900 SE 199Greentherm T9900 i 199
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.