Finding the Bad Ethernet Cable Before Blaming Wi-Fi

Check the negotiated link speed first. It costs nothing, it takes ten seconds, and it separates a cable fault from every other kind of network problem before any hardware gets bought.

A gigabit connection reporting 100 Mbps is a cable or a termination, essentially always. A connection reporting 1 Gbps and behaving badly is not.

A note on this guide: this rests on how these faults present and what each test isolates, not on cables tested here. See how we test.

The problem, as it actually presents

A wired machine feels no better than the wireless one it replaced. Or a link drops intermittently. Or a large transfer that should saturate a gigabit link crawls at a tenth of that speed.

The usual response is to suspect the router, then the network card, then the internet connection — which is expensive, in that order, and skips the cheapest possible explanation.

Why the obvious solution disappoints

Replacing the cable does fix it, when the cable is the problem. What it does not do is tell you which cable, which matters the moment there is more than one in the path: a patch lead to a wall plate, a run inside the wall, and another patch lead at the other end are three separate opportunities for a fault and one of them is not accessible.

Swapping the accessible ones and finding no improvement is how people conclude the router is broken.

What actually matters

The link speed reading is free and conclusive

Gigabit Ethernet uses all four wire pairs. 100 Mbps Ethernet uses two. A cable that has lost a conductor in one of the two extra pairs does not fail — it silently negotiates down to 100 Mbps and keeps working, which is why the symptom is slowness rather than an outage.

So: open the adapter properties, read the link speed. If it says 100 Mbps on hardware that should do 1 Gbps, the fault is in the physical path and a tester will find it in minutes. If it says 1 Gbps, stop looking at cables.

What a basic tester does and does not do

A continuity tester is a pair of small boxes with lights. One end plugs in at each end of the cable and the lights step through all eight conductors in order. A missing light is a broken conductor; lights out of order are a crossed pair. It is not a sophisticated instrument and it does not need to be, because the faults it finds are not sophisticated faults.

It does not certify a run

Crosstalk, attenuation and whether a long run will hold gigabit are certification questions, answered by instruments that cost hundreds of times more. For a domestic run of thirty metres or less, continuity plus a working 1 Gbps negotiation is the practical certification.

The options, and who each suits

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A continuity tester, for anyone who has terminated their own cable, run cable through a wall, or has more than a couple of links to check. Two boxes, a set of lights, and the fault located rather than guessed.

A known-good factory patch lead, for the simplest case — one accessible cable, one machine. Testing by substitution is legitimate when there is only one thing to substitute, and a flat cable is easier to route under a door or along a skirting board than a round one.

A small switch, when the problem turns out to be port count rather than cable quality — a machine sharing a link through a daisy chain of unknown quality.

The mistake to avoid

Blaming Wi-Fi for a wired fault, or the reverse. The two get conflated constantly, because the symptom — a game that stutters, a download that crawls — is identical either way.

The link speed reading separates them in seconds. Take it before anything is bought, and if the wired path turns out to be clean, the problem is elsewhere and fixing bad gaming Wi-Fi is the right page. If the cabling needs rethinking rather than repairing, running Ethernet around a room covers doing it without lifting a floor.