Bosch Error Code AA: Flue gas temperature too high, power limited (Tankless)
Bosch error code AA on a tankless water heater means: Flue gas temperature too high, power limited. It is also called Bosch diagnostic code AA.
Homeowner? What Bosch code AA means for you
Check, then call Your tankless water heater is reporting: Flue gas temperature too high, power limited. This is often caused by something simple. Try the safe checks below; if the code returns, book a service call before it turns into a no-heat or no-hot-water emergency.
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 AA mean?
Condensing unit flue gas above 392 F (200 C) or exhaust sensor above 145 F (63 C), so the unit limits power.
Applies to: Bosch Greentherm T9800 SE 160/199 and T9900 SE 160/199 / i 199 tankless water heaters
Common causes of Bosch code AA
- Scale in the heat exchanger
- Faulty flue or inlet water sensor
Bosch models that use code AA
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 AA: homeowner questions
What does Bosch code AA mean?
Flue gas temperature too high, power limited. Condensing unit flue gas above 392 F (200 C) or exhaust sensor above 145 F (63 C), so the unit limits power.
Is it safe to keep using my tankless water heater with code AA?
It depends on the cause. This is often caused by something simple. Try the safe checks below; if the code returns, book a service call before it turns into a no-heat or no-hot-water emergency.
Can I reset Bosch code AA myself?
You can try one reset (power off for 30 seconds, then on). If Bosch code AA comes back, the cause is still there and a licensed technician should check it.
Who can fix Bosch code AA 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.