Bosch · Tankless water heater

FA Gas leak in the gas path

Critical severity Bosch Greentherm T9800 SEBosch Greentherm T9900 SE

What the control is telling you

A leak was detected in the gas valve or manifold shutoff valves.

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

Likely causes

Leaking gas valve or manifold valve

Test and repair steps

Warning: Treat as a gas leak risk; do not run the unit.
Replace the gas valve and/or manifold, then run the gas and air tuning.

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

CE Flue blockage during operation (siphon pressure)
CF Flue blockage during operation (insufficient air)
EA Ignition failure
EB Control unit internal error
EC Flame lost during operation
EE Gas valve modulation coil disconnected
EF Wrong gas type configured
FC Touch key held over 20 seconds

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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.