MacBook Air damaged after update I'm having problems after updating my MacBook Air:


the fan noise is very loud
the computer is very slow
the battery flattens quickly


I've tried the following:


Restoring the OS
Downgrading the OS
Resetting the PRAM


I also trIed resetting the SMC but I can't do it! When I press the left shift-control-option-power buttons the computer starts up immediately, so I am sure the SMC is not reset.

On the Activity Monitor the kernel_task is over 350% CPU all the time!! 


The Boot ROM version is: MBA61.0099.B22
SMC version (system): 2.13f15  
i7 processor  
SSD 


I don't know what to do - help please.
It doesn't seem you're resetting the SMC correctly. Here’s what you do for a MacBook Air:


Shut down your computer
Keep the power cable plugged in.
Press at the same time shiftoptioncontrol (on the left side of the keyboard) and the power button
Let go
Turn your computer back on with the power button.


Let us know how you go. Many of your symptoms are listed in Apple's article on when you might want to try a SMC reset after performing a somewhat long set of other checks first to be sure it's not software or basic settings:


https://support.apple.com/en-us/HT201295