kernal_task bringing MacBook Pro to a halt - fan problem? I have acquired a 2nd-hand mid-2015 MacBook Pro which I use connected to 2x cinema displays (the laptop itself has some screen damage).

When connected up for the first time I noticed it was going insanely slow (16gb 2.8ghz w/ Radeon R9 2GB) even compared to my early-2013 model with half the spec.

Checking Activity Monitor, I noticed kernal_task was using 400% of CPU (since then I have seen over 1000%). I looked up the cause and that it was overheating so opened it up and cleaned all dust out with blowers etc. to the best of my ability - no change.

I noticed using the laptop without anything plugged in seemed to be okay, so I initially thought having 2x displayport connections to screens was doing it. Since then I've discovered (like right now) I can have 2 screens plugged in and it's okay - then sometimes I add USB devices and it's okay - but when the system really starts working and needs fans - that's when things go bad.

So I installed iStat Menus and have discovered that my right fan is not operating (constantly 0rpm even when the left is 6000+ rpm).

So I'm wondering if a) this is the root cause (and not a symptom of the cause) and b) what I can do to fix.

Even though I cleaned it I did notice at the time its movement wasn't as smooth as the left - would cleaning help or would I need a replacement? 

I've replaced MacBook components before so am quite confident in doing this if needed, but the fan isn't too cheap / available here so I'd like to know if I should expect further issues or what.

Thanks!

Mike
Do you also notice the MacBook getting much hotter on the right side than on the left under load? This would confirm that the right fan is not working, so it wouldn't be just the sensor.

The kernel_task issue is a recently discussed topic and it is often caused by high temperatures. Take a look at this active discussion.

I would replace any broken fans and distribute external hardware that draws power and the power supply evenly between right and left ports.