Smog inspections are mandatory for most vehicles, except certain diesel models predating 1998, electric, natural gas vehicles over 14,000 lbs, hybrids, motorcycles, trailers, and gasoline vehicles made before 1975. Those in areas with biennial smog checks must provide evidence of certification every other renewal. Vehicles less than six years old pay an annual smog fee for the first six years instead of biennial checks. Check your renewal notice for smog certification requirements; if needed, pay registration fees to avoid late charges even without inspection.