I have three runs of Cat 6 cable I have dragged under a floor and downstairs.
I have bought a cheap tester.

One cable, fine. The tester shows all eight lights and my laptop connects using it.

The second cable, the tester all eight lights in order, but laptop doesn't connect-I'll try re-crimping the ends on that.

The third cable is the one that bothers me-the tester doesn't light any lights. This I have hand-crimped a plug into the network switch on one end and I have put a punch-down keystone jack in a wall at the other end. I put a working patch cable into the jack and try the tester but no lights at all. I can imagine that maybe going under the floor and through joists I have managed to break a few of the wires, but hard to believe I have broken at least six of them? If even a few were still OK the tester should have at least one light? I don't think I am going crackers and testing the ends of two different cables. I can try re-crimping another plug, but I don't see how that likely really to help.

Am I missing anything? Any other reasons that would mean the tester wouldn't light any lights?

  • 1
    How hard did you pull the cables when installing?
    – Huesmann
    Jan 23 at 11:56
  • You probably have damaged the cables. Bending them backwards and forwards or at sharp angles is more likely to cause more damage rather than just pulling. It is also likely that you have a different cable end. I cant be sure how your tester works, maybe there is a short after teh cable has been damaged. Jan 23 at 12:13
  • Yeah, probably far too hard! Quite tricky getting through joists and around corners!
    – Andy Mc
    Jan 23 at 15:36
  • strip the insulation from the wires at both ends ... twist one end all together ... measure the resistance between wires at the other end
    – jsotola
    Jan 23 at 19:08
  • Were you pulling really hard to get the cable through? Because if you weren't, your problem is almost surely the termination Jan 23 at 19:55

2 Answers 2


Depending on the tester (the cheap "series of lights" ones use several different testing schemes, and generally don't provide details of what those are), and what you did wrong, there are multiple ways to get no lights without breaking the cable. i.e. if the tester is checking pairs by only checking the pairs, rather than applying voltage to one pin and connecting all the others to return, a few mis-matched pairs might give you darkness.

Multimeter is a good start, as is accepting that you might have royally borked where the wires are .vs. where they should be, and really looking carefully rather than assuming you got it right.

Yank the wires out of the punchdown, strip a little, check that there is no continuity between any of them with nothing connected. At the plug end if you think the plug end is good, punch some short wires into a jack and short one pair at a time, looking for continuity (might as well look at resistance, really) on the other end on that pair, and it should be only that pair that has continuity or non-infinite resistance. If you think the plug might be bad, chop it off and strip wires, removing it from the equation.

Which is the sort of thing a cheap tester is supposed to do for you, but some are a bit too cheap and your's isn't giving you enough feedback to point the way forward.

Breaking the wires is possible, but normally quite rare unless you did some major abuse pulling the cable.

Professionally, we avoid crimping plugs to the greatest extent possible - punchdown jacks and factory-made patch cables are MUCH more reliable than crimping plugs onto wall cables. Even worse if you're using a budget crimp tool for the crimping.

  • 3
    +1. +10 if I could for the last paragraph. Jan 23 at 15:24
  • Yeah, multimeter and connecting wires at one end seems obvious now, thanks!
    – Andy Mc
    Jan 23 at 15:37
  • 1
    And noted on the punch down jacks for next time.
    – Andy Mc
    Jan 23 at 15:38
  • 1
    @AndyMc first, make sure they are not connected. Only then do you connect them. "test for shorts - then opens". Because a short looks like continuity, it just isn't where you need it to be,
    – Ecnerwal
    Jan 23 at 15:44
  • 2
    @servant0a cheap one is fine for home use. Just avoid the "pulling too hard" mentioned in comments on the question, and do use punchdown jacks at both ends of the wall cables, as explained in the last paragraph. If you own a multimeter, skip buying a tester at all, unless you are doing a LOT of cables. I got by for many years with (first) "Does a computer connect and do 100Mbit?" followed by "Does a computer connect and do 1000Mbit.?" Unless you have money to burn, a more expensive tester is not really justified for home use.
    – Ecnerwal
    Jan 23 at 18:02

you have one known good cable, so loop that into the mystery 8 led one with a patch cable and check the combined cable with your tester - that will make it more obvious if you have the wires in the wrong order as all the LEDs are in the same room now.

The O leds cable is either damaged or possibly the termination is bad I'd be undoing the termination and checking the cable for short circuits and opens with a multimeter.

  • Thanks, good idea, I’ll try.
    – Andy Mc
    Jan 23 at 15:37

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