HVACopedia

Bosch · Tankless water heater

CE Flue blockage during operation (siphon pressure)

High severity Bosch Greentherm T9800 SEBosch Greentherm T9900 SE

What the control is telling you

The unit shut down for safety because excess pressure built up at the siphon during operation.

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

Likely causes

  • Blocked 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. Check the exhaust pipes and terminals for obstructions.
  3. Confirm the vent length is within specification.
  4. Clean or replace the air filter.
  5. Run the gas and air tuning procedure.

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

  • AD Power derate warning
  • BB Fan calibration after missed ignitions
  • BC Repeated fan calibration with similar values
  • CA Water flow above 9.2 gpm
  • CF Flue blockage during operation (insufficient air)
  • EA Ignition failure
  • EB Control unit internal error
  • EC Flame lost during operation

Save it for the next call

Star this code in the HVACopedia app and keep the whole library on your phone for jobs with no signal.

Open the app

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.