2015 5k iMac running fans at very high speed for hours

aliensporebomb

aliensporebomb

macrumors 68000
Recently ran into a situation where my 2015 5k iMac was running the fans at very high speed despite not running any processor intensive applications or rebooting.

I do run iStat menus but it was set to the "System Controlled" fan setting.

I was trying to find a reason for this when I accidentally activated Siri (which i rarely use) the fan speed dropped to a normal quiet speed.

Most peculiar to say the least.

Anyone have anything else like this occur?
 
aliensporebomb said:
Recently ran into a situation where my 2015 5k iMac was running the fans at very high speed despite not running any processor intensive applications or rebooting.

I do run iStat menus but it was set to the "System Controlled" fan setting.

I was trying to find a reason for this when I accidentally activated Siri (which i rarely use) the fan speed dropped to a normal quiet speed.

Most peculiar to say the least.

Anyone have anything else like this occur?
Click to expand...


Siri always drops down the fans so she can hear you better. Even if it means needing to down-clock the entire system to 1MHz.

GPU intensive apps lead to higher fan speeds more quickly than CPU intensive tasks. Did you see any activity? Or any activity before the fans started. Remember that the fans react to heat, not activity, so even if the activity had dropped to 0 when the fans were blasting it could've been at 100 just before

If none of this is the reason, you might want an SMC reset.
 
nambuccaheadsau said:
Hard drive been replaced or iMac opened up at all?
Click to expand...

No and no....
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casperes1996 said:
Siri always drops down the fans so she can hear you better. Even if it means needing to down-clock the entire system to 1MHz.

GPU intensive apps lead to higher fan speeds more quickly than CPU intensive tasks. Did you see any activity? Or any activity before the fans started. Remember that the fans react to heat, not activity, so even if the activity had dropped to 0 when the fans were blasting it could've been at 100 just before

If none of this is the reason, you might want an SMC reset.
Click to expand...

I think I'll reset SMC because there was no obvious cause either GPU or CPU for what was going on. Temperature in the room was air conditioned in the shade in the mid 70s fahrenheit. Thanks for the tips!
 
Update: really weird. Kernel was running upwards of 500% for no good reason and zapping pram didn't fix but resetting smc seemed to.
 
Very strange, must be a High Sierra thing since I've never seen anything like this in any previous version dating all the way back to pre 10.0. Very odd.
 
smirk said:
It's not actually that strange; what you described are typical "you need to reset the SMC" symptoms.
Click to expand...

It's not strange that the SMC reset was the fix - It's strange that the SMC state got corrupted in the first place. I mean, of course it can happen, but what triggers it is what's interesting
 
My late 2013 iMac non-fusion would fire up its fan full blast whenever it woke from sleep or booted up or quite frankly at random times...I swapped HDD for SSD and cleaned fan (Dustoff). Voila! no more fan outbursts...maybe it would help you?
 
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