Heidenhein problem

Hi ,
I am writing to try and help a friend who has a problem with an old model
Heidenhein TNC155 controller .
It is a with the Z axis and states
" Gross position error " before passing the reference marks .There is no
response from the Z axis.
This is as he told me the problem , I'm afraid at this time I haven't seen
the machine , but I thought that with so many experienced people on this
group there may be some one who has experience with this machine.
We have no experienced local agents.

This is a great group and I have been following it for some time and will
probably be involved more in the future as I have just bought a 6 yr old
mill with a Servo Products control system retrofit, I am in the process of
changing the Z azis drill from rack drive to ball screw as there was far too
much backlash. I will forward details of my changes when it is further down
the track.
Thanks in advance for any ideas with the Heidenhein
Graeme