Bosch Tankless Water Heater Error Codes

Updated October 13, 2026 · by Aurora Star Tech Inc.

44 Bosch Tankless Water Heater error codes and diagnostic codes, with causes and troubleshooting steps for each. Select a code, or find your model below.

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Bosch Tankless Water Heater error codes list

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
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
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
CA	Water flow above 9.2 gpm	Low
CE	Flue blockage during operation (siphon pressure)	High
CF	Flue blockage during operation (insufficient air)	High
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
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
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
FA	Gas leak in the gas path	Critical
FC	Touch key held over 20 seconds	Low

Find codes by Bosch model

Bosch Greentherm T9800 SE

Greentherm T9800 SE 160 Greentherm T9800 SE 199 Greentherm T9800 SEC 199

Bosch Greentherm T9900 SE

Greentherm T9900 i 199 Greentherm T9900 SE 160 Greentherm T9900 SE 199

Tankless Water Heater troubleshooting guides

No hot water All brands · Tankless Water Heater
Water temperature fluctuates All brands · Tankless Water Heater

Homeowner help: Tankless Water Heater No Hot Water: What to Check

General guidance for qualified technicians. The manufacturer's installation and service manual and local codes always take priority. Aurora Star Tech is not affiliated with, endorsed by or sponsored by any equipment manufacturer; brand and model names are used only to identify equipment and belong to their owners.