LAN Cable Link at 100 Mbps Instead of Gigabit One Pair Damaged
The PC connects, the internet works, nothing looks broken - except the link quietly negotiated 100 Mbps on a gigabit network. One pair is almost always to blame, and it is fixable in minutes.
Technical AnshuGigabit LinkPair Fault
Watch the full RJ45 crimping guide on video - every step shown live, from stripping the cable to the final tester check
This is the most common LAN cable fault in the real world, and the most missed, because nothing appears broken. 100 Mbps service uses only two of the four pairs, so a cable with a dead blue or brown pair still works - it just never reaches gigabit.
Why Gigabit Fails - Pairs in Use (Example)
Wire 1PASS
Wire 2PASS
Wire 3PASS
Wire 4CHECK
Wire 5CHECK
Wire 6PASS
Wire 7CHECK
Wire 8CHECK
10/100 Mbps needs only pins 1, 2, 3 and 6. Gigabit needs all eight wires. Any fault in the blue or brown pair quietly caps the link at 100 Mbps.
1
Understand Why 100 Mbps Is the Fallback
Fast Ethernet (100 Mbps) uses two pairs: pins 1-2 and 3-6. Gigabit Ethernet uses all four pairs at once, in both directions. When the two ports negotiate and one pair is open, shorted or split, they protect the link by falling back to the best mode that works - 100 Mbps. The link light looks healthy. Only the speed is wrong. That is why this fault hides for years on office networks.
2
Confirm It Is Really the Cable
Check the link speed where the OS reports it - Windows shows it in the Ethernet status and adapter details; a managed switch shows it per port
Connect the same PC to the same switch with a short, known-good factory patch lead. If speed becomes 1.0 Gbps, your installed cable or its plugs are guilty
Move the suspect cable to a different switch port to rule out a damaged port
3
Fix in the Order Technicians Do
Run the wire tester and look specifically at pins 4, 5, 7 and 8 - the pairs 100 Mbps never uses, where the fault almost always sits
Re-crimp the ends one at a time, testing between each. Most cases are a blue or brown conductor seated short in one plug
Check wall jacks and patch panels too - a pair punched down in the wrong slot gives the identical symptom
If the cable passes continuity but still caps at 100, suspect split pairs: re-terminate keeping twisted pairs together, untwist under 13 mm
4
Rule Out the Non-Cable Causes
One device is genuinely Fast Ethernet only - old printers, some cameras and budget switches have 100 Mbps ports by design. Check the spec before blaming cable
A damaged or very old Cat5 (not Cat5e) run can struggle at gigabit over distance - Cat5e or better is the gigabit baseline
Forced speed settings on a PC or switch port can pin the link at 100. Leave both sides on Auto unless you have a specific reason
Honest note: copying a file and "feeling" the speed is not a diagnosis. Read the negotiated link speed first. If it says 100 Mbps, no Windows setting, driver update or reboot will turn it into gigabit while a pair is faulty.
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.
Why does a faulty cable work at all instead of failing completely?
The two ports negotiate the best mode the cable can support. With one pair open, gigabit is impossible but 100 Mbps - which needs only pins 1, 2, 3 and 6 - still works, so the link comes up at 100 Mbps and stays there.
The tester shows all 8 wires connected. Can the cable still be the problem?
Yes. Split pairs pass a continuity test but fail at gigabit due to crosstalk, and a conductor can touch while being too damaged for clean signalling. Re-terminate both ends with pairs kept together, then check the live link speed.
Is it worth replacing the whole cable run?
Usually not. Nine times out of ten the fault is in a plug or a punch-down, not the cable in the wall. Re-crimp the patch lead ends first, then check the wall jack and patch panel terminations before touching the run.
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