My name is XXXXX XXXXX I am an authorized Bosch, Thermador and Gaggenau service tech and I can help you today.
This is a very common problem with this model and style machine.
If the top surface burners are working and lighting fine then that means the gas is fine to the machine as well as the gas regulator.
Now I would like for you to do one simple check for me. This will Involve removing the bottom bake plate to the oven to visually inspect the bake igniter. The remove the griddle or the drip tray under the griddle to visually inspect the griddle igniter.
In most cases when either the oven or griddle doesn't light it is usually due to a worn or damaged igniter. And on these machines if the igniter cant pull in at least 3.3 amps then the gas valve that lets gas get to the burner to ignite will not open. Which would lead you to believe the gas valve is not opening.
So I would first and most simply like for you to turn on the bake cycle at about 350 degrees for the bake in the oven and visually look at the igniter. Does it glow cherry red? Or better yet, if you have a clam on meter with amperage check on it, clamp it around one of the wires to the igniter and check the amperage to the igniter. If it is more then 3.3 then the igniter should glow cherry red in about 10 seconds. and should let the gas valve open and then light the burner.
Now if the igniter doesnt get to about 3.3 amps or if you dont have a clamp on amp meter, then if the igniter doesnt get cherry red and glows a soft orange, then we know the igniter is bad. And will need to replaced.
Do this for the griddle igniter as well. It is the same procedure.
If you need to change the igniters the part number is XXXXX for the bake igniter. and should cost about $90.
And the griddle and grill
igniters are part number 487542 and should cost about $160.
All the parts above can be ordered at either www.repairclinic.com or www.marcone.com or your local appliance parts store nearest you.
If you have any other questions please get back to me here and I will help you more. And if you run the checks on the igniter and find they are bad then just order the igniters and replace them. And please remember to rate my performance positively below by rating me with at least ok service or better rating below.
And if you still have more questions please feel free to get back to me here and I will help you more for no additional charge.