HVACopediaBosch · Tankless water heater
A0 Water temperature sensors disconnected or damaged
What the control is telling you
One of the inlet, outlet or heat-exchanger water sensors is open or unplugged.
Applies to: Bosch Greentherm T9800 SE 160/199 and T9900 SE 160/199 / i 199 tankless water heaters
Likely causes
- Loose connector
- Failed sensor or wire
Test and repair steps
- Check the sensor connections and wiring.
- Replace the sensor or wiring if the fault stays. Qualified installer only.
Models that show this code
6 models in Bosch Greentherm T9800 SE, Bosch Greentherm T9900 SE
Other codes on the same units
- A1 Cabinet air temperature above 158 F (70 C)
- A2 External water temperature sensor / aquastat fault
- A3 Flue gas temperature sensor fault
- A4 Air temperature sensor fault
- A5 Condensing unit flue temperature sensor fault
- A6 Inlet cold water temperature sensor fault
- A7 Heat exchanger hot water temperature sensor fault
- A8 Outlet hot water temperature sensor fault
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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.