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Bosch · Tankless water heater

E3 Flue gas temperature too high, burner cut off

High severity Bosch Greentherm T9800 SEBosch Greentherm T9900 SE

What the control is telling you

Exhaust temperature exceeded 167 F (75 C) on residential units (T9800 text allows 194 F on commercial), or stayed over 145 F (63 C) for 2 minutes.

Applies to: Bosch Greentherm T9800 SE 160/199 and T9900 SE 160/199 / i 199 tankless water heaters

Likely causes

  • Scale in the heat exchanger
  • Faulty flue gas sensor

Test and repair steps

  1. Descale the unit.
  2. Hold the power button down roughly 3 s to clear the stored error.
  3. Check the flue sensor resistance against the chart and replace if out of range.

Models that show this code

6 models in Bosch Greentherm T9800 SE, Bosch Greentherm T9900 SE

Greentherm T9800 SE 160Greentherm T9800 SE 199Greentherm T9800 SEC 199Greentherm T9900 SE 160Greentherm T9900 SE 199Greentherm T9900 i 199

Other codes on the same units

  • C8 Main water valve fault
  • C9 Barometric pressure sensor damaged
  • E0 Control unit internal error
  • E1 Hot water above 194 F (90 C)
  • E4 Cabinet air too hot, burner cut off
  • E5 Condensing unit flue too hot, burner cut off
  • E7 Control unit internal error
  • E8 Ionization test error

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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.