The power electronics for the motor's support control were housed within the motor in the first models (ST1 / V1 / V1.1). Due to various issues (temperature, water, humidity, and subsequent corrosion), they were first potted and eventually consolidated into a separate unit, the controller or MCU (Motor Control Unit), placed under the down tube or in the battery compartment (e.g., ST2-Belt, ST7).
Defects are quite common. As the assembly has been encapsulated in epoxy resin since around 2018, it is virtually impossible to repair.
Defects are often, but not always, visible. If MOSFETs "explode", measurement is pointless, the damage is visually obvious.
Even a defect in the charging section does not require further forensics.
Error E 210012 & E 210109
If the Omni shows error E 210012 & 210109, Stromer has instructions on what to visually check before installing a new battery. However, this is only possible with controllers that have a transparent seal.
Error E 21000E – during driving
If error E 21000 appears, it could indicate a problem with the CAN bus. To check this, go to the service menu and verify if the CAN bus is locked. Instructions on how to do this can be found in the chapter ERROR AND WARNING MESSAGES. ,E 21000E – while driving‘
No optical errors – check MOSFETs
If no optical problems are apparent, it becomes difficult to diagnose the error. A simple electrical test of the MOSFETs is possible, but only with controllers that are not potted. I am linking the document here. Testing for blown MOSFETs the company GRIN Technologies (ebikes.ca), which very easily illustrates the procedure with a multimeter.