On my gwh 1600 h ng the igniter won't spark.

http://www.ps2netdrivers.net/parts/bosch.gwh.425.hn/

On a 1600H for god sakes give the Tech the S/N and the color code on the wiring at the flow sensor that starts the hydro sparker. They changed the thing in Mfr and there are 2 different parts. And also rebuild the water valve occasionally like every 3 years as Bosch recommends even if the cover which guides the pushrod is expensive. FYI we now usually buy a new WV as it\'s more reliable than rebuilding and you get all new parts. It\'s like $125 for a WV vs $59-71 for just the WV cover. Also never run these on very hard water fro a long time or you\'ll never descale em without Whitlam and a long wash out.

Yes. Contact Bosch.
Error codes are considered start-point for troubleshoot.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

Tankless are complex electronic machines.
If on-site plumbers cannot figure out problem, then you need trained service technician with a service manual for your specific product, who can ohm-test the electronics.
Bosch will have a list of trained service technicians. And maybe Bosch has another step for fixing problem yourself.

Fix most tankless problems yourself, same day, for $330.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Before calling service technician, open following link to download typical manual, get Bosch contact information etc.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

Add a comment with your experience so others with same-similar problem can benefit.

Also take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

A lot of factors, internal and external, can cause this problem ranging from bad PC board to plumbing problem.

Since you have no error code, it could be plumbing,
http://waterheatertimer.org/pdf/No-Error-Code.pdf

Open following link for useful Bosch troubleshooting information including Bosch support and typical manual for 2700 gas tankless:
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

For folks with Rheem tankless:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Add a comment with your experience so others with same-similar problem can benefit.

Also take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

Problem could be air supply, gas supply, venting, part failure or combination of listed things.

Best place to start is manual.
http://www.boschhotwater.com/BoschModel1600H/FeaturesandSpecs/tabid/1024/Default.aspx

Typical 1600 manual shows troubleshoot starting page 19
http://www.boschhotwater.com/Portals/7/TechManuals/1600H_English_0606.pdf

Bosch has online tech support form:
http://forms.boschhotwater.com/Home/BoschTechSupportForm/tabid/614/Default.aspx

BOSCH error codes:
http://www.boschhotwater.com/HelpfulResources/TechnicalSupport/BoschAquaStarModels/2400ESTechSupport/Troubleshooting/tabid/1295/Default.aspx

Add a comment for more free help.
Also take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

The gas flame igniter is not working properly.

Go to the link below and type in your model or select it and the instructions take you through the gas pressures and so on.

I was looking for a service manual, which you apparently do not supply. I was not looking for someone who will charge me a fee to get information. I can get that info directly from Bosch for free. Thank you for answering. John

BOSH AQ 250 On demand propane water heater ran out of propane, got gas now, but get EC flashing on the display and the unit won't fire? Lost manual, any suggestions. Thanks, Jim

The manual says to hold a match near the pilot while the ignitor is trying. It says that if the unit lights the pilot orifice may be dirty or clogged. Then the manual says to check the voltage on the igniter. It should be 1.3 volts DC minimum if it is not means the electronic control box is defective.

Let me know the status ...Kevin