I encountered the same problem this summer during Vehicle Annual Check. Crown Motors informed me the problem and helped me to wash the "Fuel Injection System". Then, my car passed the check.
I think they charged me less than $2000 ... should be 1200 or 1500 ... I forgot the exact amount. Hope my information can give you an reference.
My fan told me that this problem caused by my poor driving habit. I seldom drive my car in high RPM. So, the carbon particles can't exhaust and sticked inside the car ... For sure, I am not 100% understand his say. But, spend 1k to 2k can solve the problem ... it is OK la.