Originally Posted by montr
How do I know I have carbon built-up?
For me, parts started to fail all related to air intake and emissions. It started with the mass air sensor, then a thermal sensor, followed by some related codes. After those were clear, the Service Engine Soon light returned. Certain Subaru engines show a similar pattern when the turbos start to die: metal fillings from the turbos start to foul intake parts before thrashing around in your engine. With that in mind, I did some Internet searches (most pointed here and 'fest) and when the SES returned, I asked the tech to check for carbon/oil build-up or metal filings as that many parts dying in succession sounded fishy to me.