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I wired up a ~40m long CAT6 cable following T568B pinout.

According to my tester, pins #6 and #8 are not functional.

I have re-wired both ends several times to make sure it's not my poor crimping; looks like the cable must have some damage.

I would prefer to not have to replace the cable because it's inside walls and ceilings (which is probably where it sustained damage, if any).

Is this usable?

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    Unfortunately, you need to replace the cable. Also, remember is is a Category-6 cable, not an ethernet cable, because it can be used for applications other than ethernet. Even experienced cable installers can have problems installing Catefory-6 and above cables that pass the category text suite, which requires much more than a simple wiremap. This answer explains about the basic required tests, and it does not seem you have the proper test equipment.
    – Ron Maupin
    Jan 13 at 1:10
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    In any case, if the cable installer did not also run a pull string, you can use the bad cable as a pull string, but be sure to run a pull string with it. Exceeding the maximum pull tension or minimum bend radius can permanently damage the cable. Your company should hire a certified installer who will give you a test. report on each cable. Using unqualified installers can end up costing more in the long run.
    – Ron Maupin
    Jan 13 at 1:55

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Generally, you cannot repair Cat 5/6/7/8 cabling other than at termination, and only when there enough slack left.

You could use a time-domain reflectometer to locate the fault - hopefully at one end -, remove the faulty segment and re-terminate.

If you decide to replace the cable, heed Ron's warning and hire a skilled installer. If maximum pull tension or minimum bend radius are not observed at all times, a cable can easily get permanently damaged. Also, a deployment should always be certified using a proper tester, so you know the installation is reliable.

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    Many testers have a length mode that might help locate the break. (it's capacitance, not TDR)
    – Ricky
    Jan 13 at 10:13
  • @ricky AFAIK, capacitance-based measurement is fairly inaccurate.
    – Zac67
    Jan 13 at 11:30
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If you are desperate to get some kind of link up without replacing the cable you can probably get a 100 megabit link working by re-wiring the cable in a non-standard way so that the good pairs are on the pins used by 100 megabit Ethernet.

It's not a great practice though.

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