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Bosch · Tankless water heater

E4 Cabinet air too hot, burner cut off

High severity Bosch Greentherm T9800 SEBosch Greentherm T9900 SE

What the control is telling you

Cabinet air above 176 F (80 C) or above 158 F (70 C) for 2 minutes.

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

Likely causes

  • Flue gas leak inside the cabinet or concentric vent
  • Faulty cabinet air sensor

Test and repair steps

  1. Inspect the cabinet interior and the concentric vent for combustion gas escaping.
  2. Hold the power button down roughly 3 s to clear the stored error.
  3. Check the cabinet sensor resistance 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

  • C9 Barometric pressure sensor damaged
  • E0 Control unit internal error
  • E1 Hot water above 194 F (90 C)
  • E3 Flue gas temperature too high, burner cut off
  • E5 Condensing unit flue too hot, burner cut off
  • E7 Control unit internal error
  • E8 Ionization test error
  • E9 Thermal fuse open

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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.