Last week my iMac began restarting immediately after entering my login credentials at startup.

After lots of debugging and experiments I have found that the reboot happens when it establishes a network connection. It happens with both ethernet cable and wifi. If I unplug the ethernet cable and shut off wifi, the iMac starts and I can log in with no problems.

Once it is ready, if I plug in the ethernet cable or activate wifi, it restarts almost immediately.

If I boot in safe mode, the problem does not occur, and I can use network functions.

My searching has not found anyone else describing a problem like this. Does it sound familiar to anyone here?

The iMac is a mid-2011 21.5 inch, model 12,1 core i7. It has been running without issue until this.

What I have done so far:

  • reset SMC
  • reset PRAM
  • unplugged all external accessories
  • unplugged ethernet cable
  • shut off wifi
  • booted to recovery partition, ran Disk First Aid
  • booted to recovery partition, reinstalled Mac OS 10.13.6

Logs related to power shutoff

$ log show --predicate 'eventMessage contains "Previous shutdown cause"' --last 48h
Filtering the log data using "eventMessage CONTAINS "Previous shutdown cause""
Skipping info and debug messages, pass --info and/or --debug to include.
Timestamp                       Thread     Type        Activity             PID    TTL  
2019-03-05 12:32:36.742263-0700 0xb2       Default     0x0                  0      0    kernel: (AppleSMC) Previous shutdown cause: 5
2019-03-05 12:38:45.725639-0700 0xb2       Default     0x0                  0      0    kernel: (AppleSMC) Previous shutdown cause: 0
2019-03-05 13:14:54.760970-0700 0xb2       Default     0x0                  0      0    kernel: (AppleSMC) Previous shutdown cause: 0

The above rows are repeated many times in the full log output.

https://georgegarside.com/blog/macos/shutdown-causes/ says 0 means "power disconnected" which makes me wonder if there's a glitch in the network hardware that causes an electrical problem.

System network components report

Ethernet cards:

Broadcom 57765-B0:

  Name: ethernet
  Type: Ethernet Controller
  Bus:  PCI
  Vendor ID:    0x14e4
  Device ID:    0x16b4
  Subsystem Vendor ID:  0x14e4
  Subsystem ID: 0x16b4
  Revision ID:  0x0010
  Link Width:   x1
  BSD name: en0
  Kext name:    AppleBCM5701Ethernet.kext
  Firmware version: 57765-a1.54
  Location: /System/Library/Extensions/IONetworkingFamily.kext/Contents/PlugIns/AppleBCM5701Ethernet.kext
  mDNS offload capable: c8:2a:14:28:c6:55
  Version:  10.3.1

WiFi report:

  Software Versions:
  CoreWLAN: 13.0 (1350.1)
  CoreWLANKit:  13.0 (1350.1)
  Menu Extra:   13.0 (1361.7)
  System Information:   12.0 (1300.1)
  IO80211 Family:   12.0 (1200.12.2)
  Diagnostics:  1.0 (805)
  AirPort Utility:  6.3.8 (638.9)
  Card Type:    AirPort Extreme  (0x168C, 0x9A)
  Firmware Version: Atheros 9380:
  MAC Address:  e4:ce:8f:51:8c:c8
  Locale:   FCC
  Country Code: 
  Supported PHY Modes:  802.11 a/b/g/n
  Supported Channels:   1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 36, 40, 44, 48, 52, 56, 60, 64, 100, 104, 108, 112, 116, 120, 124, 128, 132, 136, 140, 149, 153, 157, 161, 165
  Wake On Wireless: Supported
  AirDrop:  Supported
  Status:   Off

Update: I also found that the WiFi networks listed in network preferences cannot be deleted. All named SSIDs and the delete button are disabled.

  • The fact that it works normally when you boot into Safe mode means something being loaded is causing the issue. The next test you should do is create another user and see if the symptoms persist on that account. That will tell you if it’s systemwide or just your account – Allan Mar 13 at 1:42
  • Thanks Allan. I created a new user as you suggested. Same result - reboot after logging in if network is connected. – Paul S. Mar 14 at 19:02
  • Next, do a clean install on a USB flash drive (it's just for testing). See if the issue dissapears. – Allan Mar 14 at 20:34
  • Booting from a USB flash drive appears to resolve the problem. Initial tests show no problems after normal boot, both with ethernet cable and WiFi. Thanks for this suggestion. – Paul S. Mar 18 at 19:22
  • Then your easiest way to solve this is to wipe the drive and reinstall (don't just reinstall over the top). Make sure you have a backup. – Allan Mar 18 at 19:30

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