iMac won't go into sleep mode in Yosemite

Thanks a lot Mayhewcl! It woked, I just don't know exactly what it was because the first time I tried the steps one by one, it didn't seem to work. Then I quickly repeated all the steps once again (without expecting them to work), but surprise, when I put the computer to sleep it was back to normal.

Thanks a lot!

You are very welcome.

Tried everything, and the only thing that worked for me was setting the screen saver to "never". No matter what time was set for the SS, it would never allow my mini to go into sleep mode automatically. Set it to never, and then it worked.

I followed another user's suggestion to check my print que and that is what was preventing my iMac and external hard drive from going to sleep. I had been doing a bunch of printing and accidentally selected one of my printers that I wasn't using. Sure enough under system preferences in the printers and scanners section, there was a job in the que on the printer that was turned off. As soon as I cleared it out the computer went to sleep with no issues.

That was it for me - an old print job that was still in the queue. I deleted the print job and Mac sleeps again.

But this is not in Apple's official suggestions for things to check if the Mac won't sleep.

Thanks!!! Removing the printing jobs solved my problem. After weeks, now my iMac can sleep again.

Thanks to share your experience with us.

i didnt realize i had this issue until i saw my last electric bill. only suggestion that worked for me was changing sleep settings to never and manually putting the machine to sleep.

weird

Earlier suggestion to uncheck Internet Sharing in the Network control panel fixed it for me.

It was the Print Queue problem for me too. The other day I had been trying to get something to print using a Printopia 'virtual' printer and it had left some spurious jobs in a queue I never use. Deleting them cured the problem and it sleeps just fine now. How nice to hear the silence after a few seconds of the iMac tidying itself up.

Thanks for all suggestions.

wow, so many issues that lead to this. no wonder it's the top hit in gooey when searching for why iMac isn't sleeping. just wanted to let everyone know that what fixed it for me on iMac13,2 seems to have been permission verifications repair of the system volume with disk utility. sleeps fine now

I deleted a duplicate printer as well as deleting my Bluetooth iPhone connection as well as my sister's Bluetooth iPhone connection and one of those actions now lets my iMac sleep as it should!

All thanks to the following thread, please read in context...

iMac not going to sleep -- new problem

Thanks you. I have forget to disable it. Now works again.

I would have sworn I had no pending printing jobs, but I had! I deleted the culprit and my iMac now sleeps again. Thank you to all who posted!! 😀

Just thought I should post this in case it may help someone.

My imac (Mid 2011, with OSX 10.10.5) wasn't sleeping. I tried every step listed here and elsewhere, nothing worked. Then I changed the parameters of hibernatemode back and forth, and voila! now it works.

Basically I did this

sudo pmset -a hibernatemode 3

I tried to force it to sleep again, but it didn't.

Then I changed it back to 0, default mode for desktops.

sudo pmset -a hibernatemode 0

Big thanks to everyone who has written in on this post. For me, it was flushing out the printer queue - problem solved!! I followed the advice from mayhewcl to that point (and then tested out 'Sleep' before doing the scary stuff) and there were two rogue documents there.

Thanks once again to you all.