A damaged network cable is usually fixed by swapping in a known-good patch cable to confirm the fault, then either replacing the run entirely or re-terminating the RJ45 plug if only the connector is broken. Repair is only worthwhile for short, accessible cables with clean, minor damage.
Symptoms
- Link light on the switch or NIC stays off, flickers, or drops repeatedly during use
- Connection negotiates at 100 Mbit/s on a Gigabit port, indicating a broken pair
- Ping to the gateway shows packet loss, high jitter, or intermittent timeouts
- Cable tester reports an open, short, split pair, or miswired conductor
- Physical signs: crushed jacket, exposed conductors, cracked RJ45 clip, or bent boot
Common Causes
Jacket crushed by furniture or doors
Chair casters, door frames, and desks compress the cable over months. The copper pairs deform, impedance shifts, and link quality collapses even though the outside looks intact.
Tight bends below the minimum radius
Bending Cat5e/Cat6 tighter than roughly four times the cable diameter untwists the pairs and degrades crosstalk performance. Gigabit links then fall back to 100 Mbit/s or lose sync.
Broken RJ45 latch or loose contacts
A snapped locking tab lets the plug wiggle in the port, breaking one or more of the eight contact pins. This is the single most common cause of intermittent Ethernet.
Poor termination during installation
Untwisting pairs too far back, wrong T568A/B order, or a weak crimp produces a link that works at short range but fails under load or over full length.
EMI from parallel power runs
Ethernet laid alongside mains cables, fluorescent ballasts, or motors picks up interference. Symptoms look like a bad cable: CRC errors, retransmits, dropped frames.
Step-by-Step Fix
- Confirm the cable is actually the fault
Before touching tools, swap the suspect cable with a known-good patch lead of the same category. If the link comes up cleanly, you have isolated the fault. If problems persist, the port, NIC, or switch is the real culprit and no amount of recabling will help. - Inspect the full length end to end
Run the cable through your hand from plug to plug. Feel for flat spots, kinks, cuts in the jacket, and heat damage. Check both RJ45 connectors under good light: the plastic latch must be intact, the eight gold contacts must sit at equal depth, and no conductors should be visible outside the plug. - Read the link LEDs for clues
On the switch or NIC, a solid link LED with no activity, a 100 Mbit/s indicator on a Gigabit port, or a totally dark port each point to a specific fault. Amber usually means degraded link, green means full negotiated speed. Note the state before you change anything. - Test with a cable tester or certifier
Plug both ends into a basic wiremap tester. All eight LEDs should light in sequence 1 to 8 on the remote unit. An out-of-order light means a miswire, a dark light means an open conductor, and two simultaneous lights indicate a short. For longer runs, a qualifier that measures length and pair skew is more reliable. - Decide: replace the run or re-terminate
If damage sits in the middle of the jacket, replace the whole cable. If only a plug is broken on a short patch cable, re-terminate. For in-wall runs where pulling new cable is impractical, cut off the damaged plug and fit a new one, or install a keystone jack and use a short patch lead. - Re-terminate the RJ45 connector correctly
Strip about 25 mm of jacket, untwist each pair as little as possible, and arrange the eight conductors in T568B order (or T568A, but match both ends). Trim square, push fully into the plug so the jacket sits under the strain relief, then crimp firmly with a proper RJ45 crimper. Verify with the tester before returning the cable to service. - Route the new cable to avoid the original cause
Keep at least 20 cm from mains cables and fluorescent fixtures, avoid running under chair wheels, and respect the bend radius. Secure with Velcro rather than tight cable ties. If rodents were involved, use conduit or armoured cable in the affected section. - Verify link speed, duplex, and error counters
After reconnection, confirm the interface negotiates the expected speed and full duplex. On a managed switch, watch CRC and input-error counters for a few minutes under load. Zero errors on a sustained transfer means the repair is sound.
Symptom, likely cable fault, and the appropriate fix
| Symptom | Likely cause | Recommended action |
|---|---|---|
| No link light at all | Open conductor or broken plug | Test, then replace cable or re-terminate plug |
| Link drops when cable is moved | Loose crimp or cracked RJ45 latch | Re-terminate both connectors |
| Gigabit port stuck at 100 Mbit/s | Broken pair (pins 4-5 or 7-8) | Replace cable, do not repair |
| CRC errors under load | EMI or damaged pair twist | Reroute away from power, replace if persistent |
| Works short, fails at full length | Poor termination or tight bends | Re-terminate and check bend radius |
| Cable tester shows split pair | Wires in wrong T568 order | Cut plug, re-terminate in correct sequence |
Prevention
- Label both ends of every permanent run so faulty cables can be pulled without guesswork
- Use boot-protected patch cables in areas with heavy foot or chair traffic
- Keep at least one spare pre-tested patch cable per category in the comms cabinet
- Document cable routes so future work does not staple, pinch, or drill through them
FAQ
Is it worth repairing a network cable or should I always replace it?
Replace whenever the jacket is damaged, the cable is longer than a few metres, or the fault is inside a wall. Repair makes sense only for short patch cables where a single RJ45 plug is broken and the copper is intact. A new Cat6 patch lead costs less than the time spent re-terminating both ends.
Can I splice two Ethernet cables together to fix a cut?
Splicing is not recommended for Gigabit or faster links. Each junction disturbs the pair twist and impedance, causing CRC errors and speed drops. If you must join in-wall runs, use a shielded keystone coupler mounted in a faceplate rather than twisting conductors together.
How do I tell if the problem is the cable or the network port?
Swap the cable first with a known-good one. If the link still fails, move the device to a different switch port or try a different device on the same cable. This two-step swap isolates whether the fault sits in the cable, the port, or the endpoint NIC in under a minute.
Contact our IT support team for on-site cable diagnostics, re-termination, and structured cabling repairs.