The results of the commands you recommended I run are below. Still looks like bad hardware to me.
After iMac was power-cycled and with just the known good Ethernet cable plugged into the Ethernet port, Ethernet port blown out with compressed air, NO USB to Ethernet adapter plugged in:
joe@debian1:~$ date
Thu Sep 4 06:40:38 AM EDT 2025
joe@debian1:~$ sudo ifconfig
[sudo] password for joe:
enP1p3s15f0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet6 fe80::20d:93ff:fe4a:42d2 prefixlen 64 scopeid 0x20<link>
ether 00:0d:93:4a:42:d2 txqueuelen 1000 (Ethernet)
RX packets 4 bytes 5564 (5.4 KiB)
RX errors 3 dropped 7 overruns 0 frame 2
TX packets 76 bytes 12707 (12.4 KiB)
TX errors 0 dropped 27 overruns 0 carrier 0 collisions 0
lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
inet6 ::1 prefixlen 128 scopeid 0x10<host>
loop txqueuelen 1000 (Local Loopback)
RX packets 480 bytes 40266 (39.3 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 480 bytes 40266 (39.3 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
joe@debian1:~$ sudo ip link set enP1p3s15f0 up
joe@debian1:~$ sudo dhclient
sudo: dhclient: command not found
joe@debian1:~$ sudo dhcpcd
dhcpcd-10.2.4 starting
dev: loaded udev
DUID 00:01:00:01:30:4c:26:c7:00:0d:93:4a:42:d2
no interfaces have a carrier
joe@debian1:~$ sudo ip link show enP1p3s15f0
2: enP1p3s15f0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc fq_codel state DOWN mode DEFAULT group default qlen 1000
link/ether 00:0d:93:4a:42:d2 brd ff:ff:ff:ff:ff:ff
altname enx000d934a42d2
joe@debian1:~$ sudo dmesg | grep -i eth
[ 2.743937] pmac64_cpufreq: Frequency method: SCOM, Voltage method: SMU
[ 5.812401] gem 0001:03:0f.0 eth0: Sun GEM (PCI) 10/100/1000BaseT Ethernet 00:0d:93:4a:42:d2
[ 8.581065] gem 0001:03:0f.0 enP1p3s15f0: renamed from eth0
[ 9.550494] windfarm: SMU failed new fan command falling back to old method
joe@debian1:~$ sudo dmesg | grep enP1p3s15f0
[ 8.581065] gem 0001:03:0f.0 enP1p3s15f0: renamed from eth0
[ 49.467366] gem 0001:03:0f.0 enP1p3s15f0: Found BCM5221 PHY
[ 65.284839] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[ 65.284975] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[ 66.500717] gem 0001:03:0f.0 enP1p3s15f0: Link down
[ 82.308774] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[ 82.308936] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[ 90.820762] gem 0001:03:0f.0 enP1p3s15f0: Link down
[ 107.844850] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[ 107.844990] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[ 120.004718] gem 0001:03:0f.0 enP1p3s15f0: Link down
[ 139.460825] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[ 139.460964] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[ 141.892777] gem 0001:03:0f.0 enP1p3s15f0: Link down
[ 144.324829] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[ 144.324984] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[ 145.540737] gem 0001:03:0f.0 enP1p3s15f0: Link down
[ 147.972807] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[ 147.972944] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[ 150.404784] gem 0001:03:0f.0 enP1p3s15f0: Link down
[ 154.052775] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[ 154.052926] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[ 177.156744] gem 0001:03:0f.0 enP1p3s15f0: Link down
[ 192.964824] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[ 192.964960] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[ 212.420756] gem 0001:03:0f.0 enP1p3s15f0: Link down
[ 228.228763] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[ 228.228924] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[ 239.172755] gem 0001:03:0f.0 enP1p3s15f0: Link down
[ 254.980784] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[ 254.980946] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[ 267.140748] gem 0001:03:0f.0 enP1p3s15f0: Link down
[ 282.948784] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[ 282.948922] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[ 297.540760] gem 0001:03:0f.0 enP1p3s15f0: Link down
[ 315.780854] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[ 315.780996] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[ 316.996742] gem 0001:03:0f.0 enP1p3s15f0: Link down
[ 320.644820] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[ 320.644957] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[ 323.076717] gem 0001:03:0f.0 enP1p3s15f0: Link down
[ 338.884802] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[ 338.884955] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
joe@debian1:~$ sudo ifconfig
enP1p3s15f0: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500
inet6 fe80::20d:93ff:fe4a:42d2 prefixlen 64 scopeid 0x20<link>
ether 00:0d:93:4a:42:d2 txqueuelen 1000 (Ethernet)
RX packets 13 bytes 74952 (73.1 KiB)
RX errors 3 dropped 12 overruns 0 frame 2
TX packets 196 bytes 31612 (30.8 KiB)
TX errors 0 dropped 62 overruns 0 carrier 0 collisions 0
lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
inet6 ::1 prefixlen 128 scopeid 0x10<host>
loop txqueuelen 1000 (Local Loopback)
RX packets 941 bytes 78619 (76.7 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 941 bytes 78619 (76.7 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
joe@debian1:~$ sudo ip link show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: enP1p3s15f0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc fq_codel state DOWN mode DEFAULT group default qlen 1000
link/ether 00:0d:93:4a:42:d2 brd ff:ff:ff:ff:ff:ff
altname enx000d934a42d2
joe@debian1:~$
joe@debian1:~$ sudo ip addr show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet
127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host noprefixroute
valid_lft forever preferred_lft forever
2: enP1p3s15f0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
link/ether 00:0d:93:4a:42:d2 brd ff:ff:ff:ff:ff:ff
altname enx000d934a42d2
inet6 fe80::20d:93ff:fe4a:42d2/64 scope link noprefixroute
valid_lft forever preferred_lft forever
joe@debian1:~$ sudo nmcli device status
DEVICE TYPE STATE CONNECTION
lo loopback connected (externally) lo
enP1p3s15f0 ethernet unavailable --
joe@debian1:~$ sudo lspci | grep -i ethernet
0001:03:0f.0 Ethernet controller: Apple Inc. Shasta (Sun GEM)
joe@debian1:~$
joe@debian1:~$ date
Thu Sep 4 06:44:50 AM EDT 2025
joe@debian1:~$