Error u0136

Any ideas ?

Knowing what the problem with the vehicle and actually what vehicle you're referring to would be far more helpful than just asking what a fault code is.

Agree, but if you want the simple answer without the help that could be given by telling the whole story then a 2 second Google gives:

This code means that the Rear Differential Control Module (DCM) and other control modules on the vehicle are not talking to each other. The circuit most often used to communicate with is known as Controller Area Network bus communications, or simply put, CAN bus.