Cable Tester Shows One Wire Failing Diagnose by Pin Number
Seven lights good, one light missing. A cable tester does not just pass or fail your cable - the pin number it points at is a diagnosis. Here is how to read it like a technician.
Technical AnshuCable TesterFault Finding
Watch the full RJ45 crimping guide on video - every step shown live, from stripping the cable to the final tester check
Most people see one missing tester light and re-crimp both ends blindly. Stop. The failing pin tells you which pair is broken and what symptom it causes - no link, a link stuck at 100 Mbps, or dead PoE. Work by the number.
Cable Tester Result - Wire 6 Open (Example)
Wire 1PASS
Wire 2PASS
Wire 3PASS
Wire 4PASS
Wire 5PASS
Wire 6OPEN
Wire 7PASS
Wire 8PASS
An example reading: wire 6 never lights on the remote unit. That is an open conductor on pin 6 - part of the green pair in T568B.
1
Read the Tester Before Touching Anything
A missing light = open wire. The conductor is broken, too short, or not touching the plug contact
Lights in the wrong order = wires crossed during arrangement. That end must be cut off and redone in correct order
Two lights together or a flicker = short or a stray strand touching its neighbour
Note the exact pin number. It tells you which pair and which job is affected - that is your diagnosis, not a guess
2
What Each Pin Failure Usually Means
Pin 1 or 2 (orange pair, T568B) - no link at all, or constant link drops. Data pair for transmit at 10/100
Pin 3 or 6 (green pair) - no link at 10/100; at gigabit the link may fail entirely or train down
Pin 4 or 5 (blue pair) - 10/100 still works, gigabit fails or falls to 100 Mbps. Also carries PoE in Mode B
Pin 7 or 8 (brown pair) - same pattern as blue: fine at 100, broken at gigabit, and PoE Mode B loses its second leg
3
Find Which End Is Guilty
Inspect both plugs against the light: the failed conductor is often visibly short of the front wall in one plug
Flex the cable gently near each plug while testing - if the light flickers, that end has a weak crimp or a broken strand at the entry
Cut off the suspect end about 2 cm behind the plug and re-crimp. Re-crimping the same plug is impossible - plugs are single use
If both ends look perfect and the fault remains, the break is inside the cable run. Test the cable in shorter sections or replace the run
4
The Fault a Basic Tester Cannot See
A continuity tester confirms each pin connects to the same pin. It cannot detect split pairs - where wires are in the right pin order but twisted pairs were mixed during arrangement (for example pins 3 and 4 swapped as a pair). A split-pair cable passes the light test, links at 100 Mbps, then crawls or drops at gigabit because of crosstalk. If every light passes but speed is wrong, suspect split pairs or excessive untwisting, and re-terminate with pairs kept together as shown in the video.
Honest note: do not "fix" an open wire by squeezing the old plug again in the crimper. The contact teeth are already deformed and the wire did not reach - a second squeeze almost never cures it. Cut, strip fresh, re-crimp. One plug costs far less than a return visit.
Watch the Full RJ45 Crimping Video
See the crimping technique live on camera - wire order, seating depth and tester reading, exactly as done in this guide.
The tester passes all 8 wires but the link is still slow. What is wrong?
Suspect split pairs or too much untwisted wire at the plugs. A basic tester cannot see either fault. Re-terminate both ends keeping each twisted pair together and the untwist under about 13 mm, then test the real link speed.
Can I just re-crimp the same plug harder?
No. Once crimped, the contact teeth are set and a conductor that fell short stays short. Cut the plug off, strip fresh cable and use a new plug. Re-crimping the same plug is the most common false economy in cabling.
Which end of the cable has the fault?
Look for the conductor sitting short of the plug's front wall, and flex the cable near each plug while watching the tester. Flicker localises the weak end. If neither end is guilty, the break is inside the run itself.
This guide is based on the Technical Anshu RJ45 crimping video • Stuck on any step? Ask in the video comments - you will get a reply there