Thanks for for this!!! I had to look this up to clearly understand the rules. I also had no idea but still bought my downpipe anyways.
Source: https://www.dmv.ca.gov/portal/dmv/?1dmy&urile=wcm:path:/dmv_content_en/dmv/vr/smogfaq#BM2535
"Vehicles registered in areas subject to the biennial...