How to Test an Ethernet Cable (and Find a Break in the Wall)
To learn how to test an ethernet cable, run a wiremap first: connect a tester at each end and check all eight conductors map pin-to-pin. If a run is dead inside a wall, switch to TDR mode, which measures the distance to the break in metres from one end — so you can open a single small hole to repair it rather than the whole wall. This guide walks through both, in plain English, for someone who has just plastered over a cable that will not connect.
It is a horrible feeling. You ran a couple of CAT6 lines while the walls were open, boarded and skimmed everything, moved in — and the study socket has no link light. The wall is done, and you would rather not take a bolster to fresh plaster on a guess. The good news: you do not have to. With the right test you can find out whether the fault is a crushed cable, a bad plug, or the socket module, and roughly where it sits.
Step 1: Run a wiremap test
A wiremap is the core of testing any Ethernet cable. The tester sends a signal down each wire and a remote unit at the far end reports what arrives:
- OK — the wire reaches the correct pin at both ends.
- Open — a wire is broken or not seated; common after a nail, screw or a pulled-back conductor in the plug.
- Short — two wires touch, often from nicked insulation when stripping.
- Cross — two wires are swapped (for example pin 1 to pin 3); the link light may still flicker, which is why guessing fails.
- Split pair — the trickiest: continuity looks fine but the pairing is wrong, so speed collapses.
If you are not a network person, do not worry about memorising T568A and T568B — the tester shows you a simple pin-by-pin result and names the fault. That reading alone tells you whether to re-make a plug or dig deeper.
Step 2: If the run is dead in the wall, use TDR
This is the part that saves the decoration. Time Domain Reflectometry (TDR) is a feature on better handhelds — you will see it mentioned a lot once you start researching "how far along the cable is the fault". It sends a pulse down the cable and times the echo that bounces back off the break, then converts that time to a distance. So instead of "the cable is broken somewhere", you get "the fault is about 6 m from this socket".
Because TDR works from a single end, you can test the dead run from the socket you can reach and read the distance without touching the far end. Measure that distance along the cable route, and you can open one small access hole near the fault instead of chasing the whole run.
Step 3: Rule the socket module in or out
Sometimes the cable is fine and the fault is in the faceplate. Punch-down keystones can be loose or mis-terminated, so a wiremap taken from the patch end can pass while the socket still fails. Test at the faceplate with a short known-good patch lead to isolate whether the problem is the module rather than the buried cable — it is a two-minute check that can save a needless hole.
Do you need more than a £10 continuity tester?
A £10 blink-light tester confirms continuity and nothing else — useful for checking a patch lead, useless for locating a break inside a wall. The moment your problem is "the cable is buried and I need to know where the fault is", you need TDR and a proper wiremap readout. That is the honest line between the cheap box and a multifunction unit; the deeper category comparison is in our network cable tester guide, and the connector-level detail in the RJ45 cable tester guide.
A tester that finds the break, not just the fault
The Mcbazel Advanced 5-in-1 Network Cable Tester does the wiremap and the TDR distance reading in one handheld, working on Cat5e, Cat6 and Cat6a from a single end, with a clear LCD menu that non-specialists get on with. It is £76.27 inc. VAT (£63.56 ex. VAT), with free UK next-day delivery so you can be fault-finding at the weekend, a 12-month warranty, and a 4.7/5 rating across 342 reviews — including renovators who used the length reading to pinpoint a break behind plasterboard in minutes.
Dead socket behind fresh plaster? Find the break distance before you pick up a chisel.
Honest limits worth knowing
TDR distance is an estimate, not a survey mark: accuracy is best on longer runs and drops on very short cables, and the reading varies slightly with cable type, so measure to get close then confirm by eye. If the fault is a mis-punched keystone in the faceplate rather than the cable itself, a cable test can pass while the socket still fails — check the module too. And none of this applies to fibre, which needs an optical meter. If a run turns out to be crushed deep in a joist you cannot reach, it may still be a job for a cabling engineer — but you will hand them the distance, which saves their time and your money.
Frequently asked questions
Can I find a break in an ethernet cable without opening the whole wall?
Yes. A tester with TDR measures the distance to the break from one end, so you can locate it along the cable route and open a single small access point near the fault rather than exploring the whole run.
My CAT6 stopped working after plastering — is the cable ruined?
Not necessarily. A screw or nail can nick or crush a cable, but the fault is often a single pair. Run a wiremap to see which conductors failed, then use TDR to find where. Many faults are a re-termination rather than a full re-pull.
Do I need both ends of the cable to test it?
For a full wiremap you place a remote at the far end. For length and break-distance readings, TDR works from a single end — ideal when the far socket is boarded in or behind furniture.