I RECENTLY GOT THE CODES 22,23,24 ON MY IMPREZA JUST WONDERING WHAT NEEDS REPLACING

CODE 22 ''KNOCK SENSOR'' DOES IT DEFINATLEY NEED REPLACING OR IS THERE ANYWAY TO FIX IT????

CODE 23 ''MAF SENSOR'' IS THERE ANY WAY TO SALVAGE IT OR DOES IT NEED REPLACING??

CODE 24 ''Air control valve'' DOES IT NEED REPLACING OR IS IT COULD I FIX/ CLEAN IT???

any help much appreciated!

 

Problem is sometimes when you get more than 1 error code is that only 1 part may be messing up but it has a knock on effect and brings up more error codes.
I would go for the MAF first which allegedly can be cleaned with carb cleaner but i've never done it so don't know for sure if it works.