OS X won't sleep on power

Today I came across an issue where a Mac running OS X wouldn't sleep on power, but would sleep fine when on battery. The root cause was a stuck print job in the printer queue. Read on for instructions on how to troubleshoot power and sleep issues on OS X.

In OS X the pmset -g assertions command gives you a summary of the power management assertions in affect.

bob@mymac:~$ pmset -g assertions
22/04/12 12:13:18 PM AE 
Assertion status system-wide:
   ChargeInhibit                           0
   PreventUserIdleDisplaySleep             0
   PreventUserIdleSystemSleep              0
   NoRealPowerSources_debug                0
   CPUBoundAssertion                       0
   EnableIdleSleep                         1
   PreventSystemSleep                      1
   DisableInflow                           0
   DisableLowPowerBatteryWarnings          0
   ExternalMedia                           0

Listed by owning process:
  pid 36: [0x0000012c00000024] PreventSystemSleep named: "org.cups.cupsd"

Kernel Assertions: None

We see above the process org.cups.cupsd is preventing the system from sleeping. CUPS is Common Unix Print Daemon, which for OS X means our print queue.

After going to Print & Scan in system preferences we find a printer with a document that cannot be printed because the printer is offline. After deleting the document we reran pmset -g assertions:

bob@mymac:~$ pmset -g assertions
22/04/12 12:14:46 PM AE 
Assertion status system-wide:
   ChargeInhibit                           0
   PreventUserIdleDisplaySleep             0
   PreventUserIdleSystemSleep              0
   NoRealPowerSources_debug                0
   CPUBoundAssertion                       0
   EnableIdleSleep                         1
   PreventSystemSleep                      0
   DisableInflow                           0
   DisableLowPowerBatteryWarnings          0
   ExternalMedia                           0

Kernel Assertions: None

Now we see PreventSystemSleep = 0 (or False). So our system will now sleep.

Reboot and your system should sleep when on power!

— Andrew


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