How to Check an Ethernet Cable (With or Without a Cable Tester)

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Testing a LAN cable is easy with the right tools
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A LAN cable, commonly called an ethernet cable, is used to transmit data between two networked devices, such as when connecting a computer or TV to the internet router or modem. If you’re having connection problems on your devices, then the problem may be a faulty LAN cable. To test the cable, plug it into an ethernet cable tester and see if it successfully transmits a signal. If you don’t have a cable tester, there are several other troubleshooting tests to tell if the problem is the cable or your devices. Read on to learn how to test a LAN cable, with helpful testing tips from IT expert Mitch Harris.

Checking a LAN Cable

IT expert Mitch Harris recommends using an ethernet testing kit to test LAN cables. Just plug one end of the LAN cable into the TX plug on the tester, and the other into the RX plug. Switch on the tester and watch the lights as it checks the cable. If any lights don't turn on, the cable is bad.

Method 1
Method 1 of 2:

Using a Cable Tester

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  1. If your LAN cable doesn’t seem to be transmitting a signal, Harris recommends testing Cat5 and Cat6 cables with a cable testing kit. [1] Cable testing kits are easy to use, and you can find them in any electronics store, or even on Amazon . They usually come in 2 pieces: the main testing port and a receiver port.
    • Read the instructions for any product you use. While cable testers are similar, different products may have different instructions.
    • The cable tester may have both an insertion and receiver jack on one piece, meaning you don’t need a two-piece tester. Some other testers have both options, so you can run the cable into other rooms if you want to.
    • Make sure there is a battery in the tester before you use it. Most take a 9V battery.
    • If you're testing a crossover cable , get a tester that supports crossover cables.
  2. This is the insertion port. Plug either end of the cable into this port until it clicks. [2]
    • It doesn’t matter which end of the cable you insert into each port. Both ends are identical.
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  3. Again, insert the end of the cable until it clicks. This completes the connection so the tester can measure the cable’s transmission. [3]
    • If the tester has the TX and RX inputs on the same piece, plug both in. If it has a separate piece for the RX input, connect the cable there.
    • If the tester has both options for the RX input, you can choose which to use. Usually, a separate piece stretches the cable into another room to see if it transmits well over a distance.
  4. Once connected, Harris explains that you'll just need to check the sequence of lights on both ends to ensure you see the same sequence on both sides. [4] The tester will cycle through 8 positions and a ground connection, each represented by a light on the tester. Since the cable isn’t grounded, the ground position won’t light up. If all the other connections are good, each position will light up. If any lights (besides the ground) don’t light up, the cable is bad.
    • Some testers may have several different modes or switches to choose from. Refer to the instruction manual for how to set the tester if it has multiple options.
    • When you remove the ethernet cable, press down on the notch near the plug to detach it. Don’t pull it out; you could damage the machine and cable.
  5. Harris recommends replacing the cable if the lights don't illuminate, as that indicates the cable is not properly transmitting a signal. [5]
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Checking Without a Tester

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  1. A poor connection may be the first indication that your ethernet cable is faulty. If you’re using a computer, look for the connection bar on the lower right-hand side of the taskbar. If the bar is low or you have no connection, there may be a problem with the cable. If you don't have an ethernet cable tester, try using a different ethernet cable to connect the same two devices.
    • If you’re using a computer, you might get an error when you try to browse the web or reach devices on your network. If you’re using a TV, a “No Signal” message will probably appear when you turn it on.
    • Remember that this only applies if the LAN cable is connected. If you’re using WiFi , the problem may be with your router or modem. First, confirm that your computer is signed onto the network.
  2. According to Harris, this is an easy way to test whether the problem is actually the cable and not your configuration or hardware. [6] "If it works, it works," he says. [7] If you can't transmit data using either cable or your connection remains slow, it's likely a configuration or hardware problem and not a problem with both LAN cables.
    • It may take a minute for the device to receive a connection when you plug the cable in. If it takes longer than 2 minutes, the modem may have a problem.
    • Alternatively, you could also plug the cable into another device. This will indicate if something was wrong with the first device.
  3. If your internet is weak or absent , there may be a problem with the physical cable connection. Check the connectors on both devices. Push the connectors all the way in. If you hear a click, the cable wasn’t plugged in entirely. Do the same for the modem.
  4. When a network cable is connected to two functioning network interfaces, you'll usually see indicator lights on each device. If your network cards have indicator lights that don't illuminate when a cable is connected to both, the cable is likely bad. A green light usually indicates a good connection. Yellow or red lights tend to indicate signal problems. If the light is not green, then check your connection or test the cable. [8]
    • The green light may flash. This also indicates a good connection.
  5. Rips, kinks, or sharp bends could damage the cable and connection. If you’re having connection problems, physically inspect the cable. If you see any damage , the cable probably needs to be replaced.
    • LAN cables can usually bend around corners without much trouble. However, if the cable has a sharp fold, it may have internal damage.
  6. You can use a multimeter to test an ethernet cable for physical issues, such as broken wires inside. A multimeter will not be helpful in troubleshooting speed and signal quality issues. To test your LAN cable with a multimeter:
    • Place the multimeter into continuity mode .
    • Note the color-coded wires inside the cable connectors–you'll see eight wires or pins in each connector, and you'll test for continuity between the same color on each end. It may be helpful to clip the cable ends together so both connectors are facing you. [9]
    • Place one probe end on the first color-coded wire in the connector, and the other probe on the matching color in the other connector.
    • If you hear a beep, there is continuity.
    • Test all eight pairs. If any do not have continuity, replace the cable.
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Expert Q&A

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  • Question
    How do I know if my LAN cable is not working?
    Mitch Harris
    IT Expert
    Mitch Harris is an IT Expert based in the San Francisco Bay Area. Mitch runs his own IT Consulting company called Mitch the Geek, helping individuals and businesses with home office technology, data security, remote support, and cybersecurity compliance. Mitch earned a BS in Psychology, English, and Physics and graduated Cum Laude from Northern Arizona University.
    Mitch Harris
    IT Expert
    Expert Answer
    Check the cable with a testing kit. With this kit, each individual copper line is tested within your cable via a series of lights. If any of these lights stay dark, you'll know that your cable isn't working.
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      References

      1. ↑ Mitch Harris. Consumer Technology Expert. Expert Interview. 23 June 2021.
      2. ↑ https://youtu.be/7pdJ2aVrMPQ?t=140
      3. ↑ https://youtu.be/7pdJ2aVrMPQ?t=150
      4. ↑ Mitch Harris. Consumer Technology Expert. Expert Interview. 23 June 2021.
      5. ↑ Mitch Harris. Consumer Technology Expert. Expert Interview. 23 June 2021.
      6. ↑ Mitch Harris. Consumer Technology Expert. Expert Interview. 23 June 2021.
      7. ↑ Mitch Harris. Consumer Technology Expert. Expert Interview. 23 June 2021.
      8. ↑ https://www.pcworld.com/article/2835564/when-your-ethernet-wont-connect.html
      9. ↑ https://www.youtube.com/watch?v=84yhs3FilKc

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