Hi, I have a Bosch WR250 tankless gas instant water heater. My problem is i am not getting a spark at the pilot when the hot water is turned on,the red light flashes after 10sec. I can here the (water/spark)generator turning but i get nothing. I have checked all electrical connections and they seem fine, any suggestions please?

possible faulty pressure switch connected to the water line and that is failing to turn on the main gas tap solenoid
best have it checked by an accredited gas fitter or service center for the make

Yes you need to delime tankless each year.
http://waterheatertimer.org/pdf/Bosch_Descaling-heat-exchanger.pdf
http://waterheatertimer.org/pdf/Rheem-De-lime-tankless.pdf

Contact Bosch for exact repair information. My records show following number for tankless gas: 800-642-3111
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

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What leads you to suspect that pilot blew out instead of another problem?
Fact that pilot will not relight, indicates other factors at play.

Open following link for Bosch troubleshooting resources including manual, and Bosch contact info:
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

Best place to start is your manual.
http://waterheatertimer.org/pdf/Bosch_10P-installation.pdf

1) Manual recommends 'cleaning by authorized person, never less than once a year.'
Tankless water heaters, and most newer tank-type gas water heaters, are highly technical and finicky devices that require continual maintenance, with tankless requiring 'yearly' maintenance while tank-type require 'periodic' maintenance.
2) Troubleshoot in manual says 'no gas supply' or 'blocked pilot' replace pilot injector

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Check and be sure gas knob is in the pilot lighting position and pushed in. Try to manually light with a match. Make sure gas **** is in the on position. Posible air in the gas line. Bled off air. gas valve or regulator has failed..

I had a similar problem with mine and it turned out to be a plugged air vent sceen. This is the small dome like sceen on the gas valve. I vaccumed out the unit and have not had the pilot go out in over a year.

I have the same problem with the same model. The problem has been identified by a friend of mine, correctly, I think.
This is a very dangerous problem involving Bosch's much touted safety feature, ironically enough. IT IS THE FLAME SENSOR IN THE GAS VALVE. The flame sensor is supposed to confirm that the pilot has lighted BEFORE the main gas line sends gas to the main burners, after which the pilot shuts off. The pilot light is not coming on, the sensor that shouldn't let gas fto the main burners is failing and the main gas is flowing and ignighting from the spark that is supposed to light only the pilot in a big explosion.
This is the 2nd unit of this model that has failed. I think Bosch should do a recall

sounds like blockage pilot jet nipple needs cleaned best of luck

may require gas to to be purge leave to expert

Thermo coupler may be defective. it controls the main gas valve by closing main if pilot is out. make sure thermo coupler bulb is positioned over and near tip of pilot flame, but not in the flame. Thermo couplers are inexpensive and easy to change out.

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