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Bosch · Tankless water heater

CF Flue blockage during operation (insufficient air)

High severity Bosch Greentherm T9800 SEBosch Greentherm T9900 SE

What the control is telling you

The unit shut down for safety because of insufficient combustion air during operation.

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

Likely causes

  • Blocked intake or exhaust
  • Vent too long
  • Dirty air filter

Test and repair steps

  1. Hold the power button down roughly 3 s to clear the stored error.
  2. Look for anything restricting the combustion air inlet or the exhaust pipes and their terminals.
  3. Confirm the vent length is within specification.
  4. Clean or replace the air filter.
  5. Run the gas and air tuning procedure.
  6. The water supply can also be shut off and reopened to reset (limited to 3 times per hour); allow up to 3 minutes for start-up.

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

  • BB Fan calibration after missed ignitions
  • BC Repeated fan calibration with similar values
  • CA Water flow above 9.2 gpm
  • CE Flue blockage during operation (siphon pressure)
  • EA Ignition failure
  • EB Control unit internal error
  • EC Flame lost during operation
  • EE Gas valve modulation coil disconnected

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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.