Interesting to note is that 1997-older diesel trucks/cars did not and still do not require any smog testing at all.
"Currently, smog inspections are required for all vehicles except diesel powered vehicles 1997 year model and older or with a Gross Vehicle Weight (GVWR) of more than 14,000 lbs, electric, natural gas powered vehicles over 14,000 lbs, motorcycles, trailers, or gasoline powered vehicles 1975 and older."[0]
"Currently, smog inspections are required for all vehicles except diesel powered vehicles 1997 year model and older or with a Gross Vehicle Weight (GVWR) of more than 14,000 lbs, electric, natural gas powered vehicles over 14,000 lbs, motorcycles, trailers, or gasoline powered vehicles 1975 and older."[0]
[0] https://www.dmv.ca.gov/portal/dmv/detail/vr/smogfaq