Problems can range from bad check valve to crossover in shower faucet, to bad PC board to tripped breaker. That's a pretty broad range.http://waterheatertimer.org/pdf/No-Error-Code.pdf
Open link below for a few self-help remedies and many resources; contact service technician and contact Bosch, read manual cover to cover, make sure tankless receives yearly maintenance as outline in manual and suggested by service technician.
Open following link for Bosch tankless troubleshooting resources, including Bosch contact information, typical manuals, error codes, tech support, parts, fuel-saver calculator, service bulletins, water heater forum, fixya links, disclaimers, and links for finding service technician.http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html
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