Before calling service technician, open following links for troubleshoot resources:http://waterheatertimer.org/pdf/Bosch_E1_troubleshooting.pdf
Before installing tankless water heater, read the service manuals and
product manual to make sure you have testing skills and equipment needed
to repair tankless gas water heater, and that you are prepared to buy
expensive parts when needed and do the recommended yearly and monthly maintenance as outlined in manual.
Also check that area has qualified service technician, and that person
carries parts on-hand, and is certified for your model number.
Keep Tankless manufacturer contact information on-hand, and be skeptical
of promises of unlimited hot water, and effortless long-lasting water
heating, and saving money. The only folks saying this are the folks
selling the product, and not their customers.
Fix most tankless gas water heaters yourself, same day, all brads for about $350.