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Catalina 10.5.7 on an iMac 27" 2020.

From one day to another the Ethernet connection to the router has stopped. It went from a green to a red dot. Whatever I try the message stays always the same: Either the cable for Ethernet is not plugged in or the device at the other end is not responding.

Checked the cable with a MacBook, no problem.

Checked all ports in the router with a MacBook, no problem.

Installed a "clean copy" of Catalina on an external drive and booted the iMac from the external drive: no Ethernet.

BUT: Booting a Macbook from the same external drive: Ethernet is alive

Tried also this: I started up from macOS Recovery on the iMac, using Recovery > MacOS Utilities > Get Help Online: Ethernet is not working there either. Only WiFi.

Conclusion: damaged ethernet port on the iMac just like this?

Or is there a way to make Ethernet working again? What else can I do?

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Yes, according to your tests and observations it sounds like the Ethernet port in the iMac is physically damaged.

This could happen due to some sort of electrical issue such as a transient or similar spike of electricity. Perhaps it was lightning weather around the time it happened - perhaps it was a malfunction in the router - perhaps it was just tired hardware in the iMac.

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  • I have been using also Apple Diagnostics (former Apple Hardware Test). The result was: No errors. What then is this diagnostic tool good for?
    – MacUser12
    Commented Jun 16, 2021 at 5:23
  • If the issue is with the physical socket, then unless it runs actual tests over a connected link [which requires a special loopback ethernet plug], it won't be able to see that.
    – Tetsujin
    Commented Jun 16, 2021 at 5:32
  • Does replacing the ethernet port on this mac means to change the motherboard?
    – MacUser12
    Commented Jun 16, 2021 at 6:43
  • Yes, most likely - at least if you do an official repair. You could consider connecting your ethernet through other means - for example a Thunderbolt ethernet adapter or a USB-ethernet adapter.
    – jksoegaard
    Commented Jun 16, 2021 at 9:00

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