run diagnostics

Please refer to https://support.apple.com/en-us/HT202731

However, you also should do the following.

Please navigate to the Mac App Store or navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

For instructions on how to download your EtreCheck report and attach it to your reply to this message please click https://discussions.apple.com/docs/DOC-250000211

My iMac is running slow. I've run EtreCheck and it tells me my hard drive may be failing and the iMac is obsolete. Hoping I may be able to get rid of some obsolete software and files to perhaps get some additional life. It's been a great computer, but I can't really afford to replace it at the moment.

I’m running diagnostics on my 2017 iMac because it’s slow and the wheel is frequent but it’s gone into sleep mode and I can’t reopen the screen to check how it’s doing! Any suggestions please?

I was too hasty in my post! The screen came back when diagnostics had finished! All good.