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Bosch · Tankless water heater

E8 Ionization test error

Medium severity Bosch Greentherm T9800 SEBosch Greentherm T9900 SE

What the control is telling you

The control failed its flame ionization self-test.

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

Likely causes

  • Control board fault

Test and repair steps

  1. Replace the electronic control unit.

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

  • E3 Flue gas temperature too high, burner cut off
  • E4 Cabinet air too hot, burner cut off
  • E5 Condensing unit flue too hot, burner cut off
  • E7 Control unit internal error
  • E9 Thermal fuse open
  • F2 Control unit internal error
  • F3 Control unit internal error / configuration
  • F7 Control unit internal 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.