Check around in the oven under compartment, especially on the side walls. There should be a plastic bag with a service & diagnostic technical bulletin taped to the wall. It will have the error code list and a step-by-step diagnostic procedure.
If the list isn't there, get the model & serial numbers and call Bosch tech support. They are very helpful & will e-mail you a copy of the diagnostic paper.
PS: On on-line search for the error code may give you false reasons.
The code is different for different model numbers.
Example: E011 might mean faulty door switch on one Bosch oven model and faulty temperature control on another. You need the model and serial number to verify.
Hope This Helps!
Paul