The easiest way to check e-units for proper function is to wire it up to a motor, connect track power to it and cycle the e-unit. It should cycle consistently and the motor should run and reverse properly with no dead spots on the e-unit that do not function properly.
A couple of you tube videos here and here explain e-unit servicing, although I would recommend the proper E-unit Spreading Tool to open the unit.
Replacement parts are available from numerous online suppliers.
Larry