C2 Flue blockage at start-up

What the control is telling you

Same cause as C1: too little combustion air at start-up; the unit shuts down for safety.

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

Other codes on the same units

• A7 Heat exchanger hot water temperature sensor fault
• A8 Outlet hot water temperature sensor fault
• A9 Output power too low
• C1 Flue blockage at start-up
• C3 Flow sensor sees no recirculation
• C5 Bypass water valve fault
• C7 Fan fault
• C8 Main water valve fault