Bosch Error Code A2: External water temperature sensor / aquastat fault (Tankless)
Bosch error code A2 on a tankless water heater means: External water temperature sensor / aquastat fault. It is also called Bosch diagnostic code A2.
Homeowner? What Bosch code A2 means for you
Usually minor Your tankless water heater is reporting: External water temperature sensor / aquastat fault. This code is usually informational or minor. The safe checks below often clear it; call us if it keeps showing up.
Safe things you can check yourself
- Check that gas reaches the house: other gas appliances work, or the propane tank is not empty.
- Make sure the heater is plugged in and its breaker is on. In winter, keep the power on: it protects the unit from freezing.
- Outside, make sure the intake and exhaust pipes or vent cap are not blocked by snow, ice, leaves or a nest.
- Check that the hot and cold water valves under the heater are fully open.
- If you have a remote controller, turn it off and on once. If the code comes back, call a technician.
Call (647) 657-2977Book a service call
Licensed technicians serving Vaughan, Toronto and the GTA.
What does Bosch error code A2 mean?
The external tank sensor is disconnected, damaged or badly placed.
Applies to: Bosch Greentherm T9800 SE 160/199 and T9900 SE 160/199 / i 199 tankless water heaters
Common causes of Bosch code A2
- Sensor not fully seated in the tank well
- Wrong recirculation setting
- Failed sensor or wiring
Bosch models that use code A2
Greentherm T9800 SE 160Greentherm T9800 SE 199Greentherm T9800 SEC 199Greentherm T9900 i 199Greentherm T9900 SE 160Greentherm T9900 SE 199
Other Bosch Tankless codes
All Bosch Tankless Water Heater error codes →
Bosch code A2: homeowner questions
What does Bosch code A2 mean?
External water temperature sensor / aquastat fault. The external tank sensor is disconnected, damaged or badly placed.
Is it safe to keep using my tankless water heater with code A2?
Usually yes. Code A2 is normally minor, but if it keeps coming back have it checked.
Can I reset Bosch code A2 myself?
You can try one reset (power off for 30 seconds, then on). If Bosch code A2 comes back, the cause is still there and a licensed technician should check it.
Who can fix Bosch code A2 near me?
Aurora Star Tech Inc. repairs Bosch tankless water heaters in Vaughan, Toronto and across the GTA. Call (647) 657-2977 or book a service call online.
Homeowner help: Tankless Water Heater No Hot Water: What to Check
Troubleshooting guides
- No hot water All brands · Tankless Water Heater
- Water temperature fluctuates All brands · Tankless Water Heater
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. Written in our own words as a summary; always check the original manual.
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.