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:


What could that be? I found nothing on the internet.

  • No, I don't have any freezes.
    – fgroeger
    Commented May 31, 2015 at 11:39

2 Answers 2


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.


The 4SNS error with a V after the ':' indicates an error with a voltage sensor somewhere in the machine. This is an obvious hardware problem that may or may not be serious. The only symptom you experience is hearing the fan turn on briefly, but other problems may be occurring in the circuitry that may be deleterious to the health of your Mac. Have it checked out.

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