Why does my iMac keep waking up (I've checked the usual suspects)?

Question

Starting a couple of weeks after I got my new iMac (3.4 GHz Core i7) -- so about two or three weeks ago -- it started spontaneously waking from sleep (or turning on its screen, if it wasn't asleep), every few minutes. I can't figure out why.
I've checked to see if there are spurious input sources, and as far as I can tell, there aren't.
Wake on Network Access is turned off (even though I actually would prefer it to be on, I've turned it off while trying to sort this out.
I don't have, as far as I can tell, any new apps or tools installed that would be waking it up.
If I quit all my applications, it doesn't seem to affect anything.
The wake reason string when I search logs is "EHC1". That doesn't tell me much.
Any ideas on what else I can check?

Answer 1

Test after each of the following steps that you haven’t already tried:
Step 1
 ▹ System Preferences ▹ Print & Scan ▹ Open Print Queue...
Check all printers for unfinished jobs. Deal with any you find.
Step 2
 ▹ System Preferences ▹ Energy Saver ▹ Power Adapter ▹ Wake for network access: uncheck. You may need to unlock the preference pane by clicking the lock icon in the lower left corner and entering your administrator password.
Step 3
 ▹ System Preferences ▹ Energy Saver ▹ Schedule… ▹ Start up or wake: uncheck
Step 4
Disconnect all wired peripherals except keyboard, mouse or trackball, and monitor, if applicable.
Step 5
Launch the Activity Monitor application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Activity Monitor in the page that opens.
Select All Processes from the menu in the toolbar of the Activity Monitor window, if not already selected. Enter “powerd” (without the quotes) in the "Filter" text field. Select the powerd process and click the Quit Process button in the toobar (stop-sign icon.) In the sheet that opens, click Force Quit. You’ll be prompted for your administrator password. The process will be relaunched immediately.
Step 6
Boot in safe mode and log in. The instructions provided by Apple are as follows:
Be sure your Mac is shut down.
Press the power button.
Immediately after you hear the startup tone, hold the Shift key. The Shift key should be held as soon as possible after the startup tone, but not before the tone.
Release the Shift key when you see the gray Apple icon and the progress indicator (looks like a spinning gear).
The login screen appears even if you usually log in automatically. You must know your login password in order to log in. If you’ve forgotten the password, you will need to reset it before you begin.
Note: If FileVault is enabled under OS X 10.7 or later, or if a firmware password is set, you can’t boot in safe mode.
Safe mode is much slower to boot and run than normal. Don’t launch any applications at first. If sleep still doesn’t work properly, back up all data and reinstall the OS. After that, if you still have the issue, make a “Genius” appointment at an Apple Store to have the machine tested.
If sleep now works as expected, go on to the next step.
Step 7
Still in safe mode, launch the usual set of applications that are running when you have the problem, including your login items, one at a time, testing after each one. Some applications may not work; skip them. You might be able to identify the cause of the problem this way.
Step 8
If sleep is still working after you’ve launched all the usual applications, reboot as usual (not in safe mode) and test again. If sleep still works, you’re done, at least for the moment.
If you still have the sleep issue after booting out of safe mode, post again.