1) BOSCH website shows error codes.
FO does not appear: http://www.boschhotwater.com/HelpfulResources/TechnicalSupport/BoschAquaStarModels/2400ESTechSupport/Troubleshooting/tabid/1295/Default.aspx
2) I do not see FO error code listed on Internet search
Other FO questions at fixya seem to have no answer:
3) At this point, I would suggest calling a service technician.
While waiting for service, read manual cover to cover several times.
4) Your manual is valuable resource, and shows the word 'maintenance' and 'troubleshoot' frequently.
Tankless are maintenance heavy, and should be de-limed each year.
U-tube search "delime tankless water heater."
5) Also explore Bosch website for your manual and other valuable information.
6) When service tech arrives, ask that person for tips and tricks about your unit that will help you maintain your water heater.
7) All costs are energy. Install DIY tank-type gas water heater and avoid the whole headache.http://waterheatertimer.org/How-to-install-gas-water-heater.html