TAC Generator error is generated from either the encoder or the drive. There is a jumper on the drive that you can set to bypass the tac generator error if you wish to temporarily run however I would not recommend it for Z-axis. You may be able to swap the top boards on the drive with another axis drive. But you must check that they are the same part numbers as well as set any jumpers on the boards. Since the Z axis has a brake and gravity will make the head drop like a rock I would recommend putting a block of wood under the head and move it down to just above it as a precaution. I have never had any problems before but it only takes a few minutes just incase. This will help you determine if its the top board on the drive or an encoder problem.
Hope this helps