Question posted under Boasch tankless. Model number not known.
Before calling service technician, open following link for Bosch troubleshooting resources, including phone and contact for Bosch, error codes, and possible manual, plus hours of reading and research:http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html
As you know from reading manual, tankless water heaters require yearly maintenance by trained service technician, or by homeowner. This action would check gas and pressure, venting draft, condensation drain, burner and combustion parts, igniter and-or pilot light, electrical connections, air intake and general condition of plumbing.
Problem you describe does not include error code.
Open following link for possible help with general plumbing problem:http://waterheatertimer.org/pdf/No-Error-Code.pdf
Add a comment with model number that appears on side of unit, and I can directly locate possible service bulletins and further troubleshoot resources available at Bosch for possible do-it-yourself diagnosis and repair.