Apple iMac Hardware Test: 4SNS : VH1R I have some weird fan issues with my iMac. It sometimes goes to maximum speed but only for a couple of seconds and then goes back to normal. Without doing any heavy processing. The hardware test tells me there's an error:

4SNS/1/40000001:VH1R-0.000


What could that be? I found nothing on the internet.
CNet has an article on the codes returned by the diagnostic test. http://www.cnet.com/news/how-to-invoke-and-interpret-the-apple-hardware-tests/

According to that VH... means voltage on the hard disk.

At a big guess you might have a dodgy connection to your hard-disk. If its intermittent then the voltage may drop, triggering the safety feature of the fans running at max speed.