RJ45 Line 6 not connected

Hi!

I have a client whose network has been working for over a year without any
problems. Just lately they keep getting the network disconnected message, it
eventually works again.

As it is hapenning on all their PC's I suspect it being the HUB/Switch and I
am going to replace it as a try.

However, what has me stumped is that when I put a RJ45 cable tester on cable
the runs to the switch, they all show that line 6 is not connected. (I have
one of those cable testers that cycle through eight led lights, it misses
led 6.)

When I checked the wiring documentation, it says lines 1,2,3 & 6 are used.
Line 6 is listed as TX-. My question is how is the networking working?

Thanking you in advance.

Regards
Gitendra