Tasty basically anypart not CARB approved or could be considered for "racing" is technically not allowed and or upto the opinion of the officer. If it was available from the factory no issues. The biggest thing is if you don't have reciepts on hand they assume the aftermarket parts are "stolen" and can confiscate your car costing you tow/storage until you "prove" your innocence.

California Crackdown on Modified Cars
$5 million in federal money will fund a Sacramento, California effort to stop drivers in modified cars.

Police in Sacramento, California announced Wednesday that they would use $5 million in federal money to begin cracking down on auto enthusiasts who modify their vehicles. The money will be used to form an undercover "Drag-Net" unit to stop motorists who appear to be driving modified cars.

The concept, which originated in San Diego in 2001, has been spreading throughout California generating significant revenue for the state and local departments. In Santa Fe Springs, for example, twelve officers on Drag-Net duty issued 300 citations and impounded 50 vehicles in just one weekend. Several cities have drag-racing ordinances that allow police to auction off seized cars and keep the profits.

Under Drag-Net, San Diego officers come to train other departments how to look out for what they believe to be tell-tale signs of illegal modification such as window tinting, large spoilers, extra gauges or racing stickers. Police say this gives them probable cause to stop and inspect a vehicle and its engine compartment.

In practice, "excessive exhaust noise" tickets are the most common violation. California law does not require police to measure sound levels objectively. Instead, according to the California Highway Patrol, the "citation is based on officer's judgment."

http://www.thenewspaper.com/news/05/576.asp
Drivers of stock vehicles that come from the factory with some of the characteristics of modified cars have experienced harassment under this provision. One such motorist complained on an enthusiast website that the California Highway Patrol was using these programs to make "driving while Asian" a crime, pointing out that the department's own website has several pages dedicated to Asian involvement in street racing and "vehicle modification."

Those receiving a vehicle modification "fix-it" ticket must visit a California Bureau of Automobile Repair office and pay a $35 fee to have their car inspected. If the car fails, a judge can impose another fine of up to $2000 for failing to meet California emissions requirements