First, look carefully at the jacket and both RJ45 plugs. Cuts, kinks, crushed pins. Swap switch ports or drop in a known-good cable to confirm where the fault actually sits. Plug damage only? Cut back and fit a new connector. Broken copper mid-run? That's a replacement job.
Symptoms
Link light on the switch or NIC is flickering, or stays dark after plugging in
Connection falls back to 100 Mbps when it should hit 1 Gbps, or drops packets mid-transfer
Intermittent disconnects that happen to clear up when you wiggle the cable near one of the plugs
Tester shows an open, short, or split pair on one or more conductors
Something visible: a bent boot, exposed copper, a flattened jacket section, or a chewed run
Common Causes
Broken Conductor Inside the Jacket
A cable that flexes repeatedly at a doorway, gets pinched under a chair caster, or is lashed tight against a cable tie can eventually snap one of the eight internal conductors. The connection often still comes up, just at a reduced speed. That's what makes it hard to catch: everything looks fine until you go looking for why performance is poor.
Damaged RJ45 Plug or Loose Contact
Without a retention clip, the plug can back out just far enough to lose contact. Bent gold pins do the same thing. It's the most common reason a cable tests bad, and a quick fix once you've spotted it.
Kinks, Crush Points, and Tight Bends
Ethernet cable needs a minimum bend radius of at least four times its own diameter. A furniture leg sitting on the run, a staple hammered in too hard, a section tucked under carpet: each crushes the internal pairs and shifts impedance. A visual check won't find that. A tester will.
EMI from Parallel Power Runs
Cat5e and Cat6 running beside mains cabling or near fluorescent ballasts will pick up noise. The cable can be mechanically perfect and still throw CRC errors continuously.
Rodent or pet damage
Don't overlook rodent damage either. Mice and rabbits chew straight through PVC jackets and into the copper. The bite marks are often tucked behind server racks or up inside ceiling voids where nobody looks.
Step-by-Step Fix
Inspect Both Ends and the Full Cable Length
Pull the cable from the switch or wall jack and work it through your hands slowly. Check each RJ45 plug: missing retention clips, bent pins, cracked housings. Walk the full jacket length for cuts, staple marks, and flattened spots. Damage near the plug is fixable. Mid-run damage is almost always a cable replacement.Confirm the Fault by Swapping Cable and Port
Swap in a known-good patch cord of the same category. Link comes up clean? Culprit found. Move that working cable to a different switch port as well, to rule out a dead jack. Work through it: cable first, then port, then NIC. Skip that sequence and you risk replacing the wrong component entirely.Test Wire Map and Length with a Cable Tester
On a basic continuity tester, LEDs 1 through 8 should light in sequence at both ends. Missing or out-of-order flashes mean an open, a short, or a miswire. For structured cabling, use a certifier that reports length, NEXT, and return loss. It will pinpoint exactly how far from the plug the fault sits.Check Switch Counters for CRC and FCS Errors
On a managed switch, run show interfaces on the affected port. CRC, alignment, or runt counters climbing? That points to a marginal cable or EMI pickup, not a hard break. Clear the counters, push traffic with iperf3, and check back within a minute to see whether those numbers are rising again.Re-terminate the RJ45 Plug for Minor Plug Damage
Cut the bad plug off about 5 cm behind the boot. Strip 25 mm of jacket, keep the pairs as twisted as possible right up to the point of insertion, and lay the wires out in T568B order: white-orange, orange, white-green, blue, white-blue, green, white-brown, brown. Trim ends flush, push them fully into a fresh plug, and crimp firmly.Verify the Repaired Cable End-to-End
Run the tester again on the full cable. All eight conductors should map correctly with no split pairs showing. Plug back in, confirm the link negotiates at full gigabit, then run a file transfer for a few minutes. If it holds, you're done.Replace the Run When the Copper Is Broken Mid-Cable
Twist caps and electrical tape destroy pair geometry. Splice solid-core Ethernet that way and it will fall apart at gigabit, sometimes immediately, sometimes weeks later. For any break outside the plug zone, pull fresh Cat6 or Cat6a. A keystone-to-keystone coupler with short factory patch cords at each end is also acceptable.
Common cable symptoms mapped to likely cause and fix
Symptom | Likely cause | Action |
|---|---|---|
No link light at all | Broken conductor or dead plug | Swap cable, then re-terminate or replace |
Link trains at 100 Mbps, not 1 Gbps | One pair open or shorted | Test wire map, re-terminate the closer plug |
Intermittent drops when cable is moved | Loose crimp or cracked plug housing | Cut back and crimp a new RJ45 |
High CRC errors on switch port | EMI, kinked pair, or marginal cable | Reroute away from power, retest with certifier |
Tester shows split pair | Wires crimped in wrong T568B order | Recut and re-terminate using correct pinout |
Chewed jacket, exposed copper mid-run | Rodent damage | Replace the full cable run |
Prevention
At each end, leave a short service loop so the plug itself is not bearing the strain of the run
Keep Ethernet runs at least 15 cm clear of parallel mains cabling and fluorescent fixtures
Fit booted RJ45 plugs on any patch cord that gets moved around regularly
Label both ends of every permanent run. When something fails, you want to find it in seconds, not spend half an hour tracing which unlabelled cable is the one.
FAQ
Can I splice a broken Ethernet cable with electrical tape or wire nuts?
No. Ethernet depends on exact twist rates and controlled pair geometry to hold gigabit speeds. Disturb those pairs by even a centimetre during a splice and you're stuck with crosstalk and return-loss issues that no amount of tape will fix. Fit a proper RJ45 coupler with short patch cords on each side, or just pull a new run. Past 100 Mbps, a fresh cable is the only fix worth trusting.
How do I decide between re-terminating and replacing the cable?
If the damage is confined to the last few centimetres at the plug, broken clip, cracked housing, bent pins, re-terminate. Jacket cut mid-run? Copper showing? Tester flagging a fault more than about 30 cm from either end? Pull a replacement. Solid-core in-wall cable does not tolerate mid-run repairs. Don't attempt it.
Which cable category should I use when replacing a damaged run?
Cat6 is the right call for most home and office work. It handles 1 Gbps comfortably and can push 10 Gbps over shorter distances. Runs past roughly 40 metres, or thinking about 10 Gbps later on? Go Cat6a from the start. Cat5e still pulls gigabit fine, but the price gap over Cat6 has nearly disappeared.