You should read this as it might pertain to your situation:
Some firmware updates for Intel-based Macs may not be displayed automatically using Software Update. If they do not appear in Software Update, they must be manually downloaded from the Apple Downloads site. If you are unsure whether your computer needs a particular update, simply download and open the update installer. The installer will alert you if the firmware update is already installed or not needed.
Some firmware updates for Intel-based Macs may not be displayed automatically using Software Update. If they do not appear in Software Update, they must be manually downloaded from the Apple Downloads site. If you are unsure whether your computer needs a particular update, simply download and open the update installer. The installer will alert you if the firmware update is already installed or not needed.
Check your computer's EFI Boot ROM or SMC firmware version.
- From the Apple () menu, choose About This Mac.
- Click More Info.
- For OS X Lion and OS X Mountain Lion, click System Report. For Mac OS X Snow Leopard and earlier, proceed directly to step 4.
- Open the Hardware section to reveal Model Identifier, Boot ROM version, and SMC version numbers.
- The version information is in the pane on the right.
You might want to manually download and install THIS version of the file to ensure that you are getting the right one, IF you are sure you have a model version iMac 12.1: