Failing to show proof of financial responsibility, meaning insurance coverage, is a violation of California Vehicle Code Section 16029. Violating this code could result in fines up to $500 plus penalties, impoundment of the vehicle and suspension of the driver's license. If the vehicle is impounded the owner will have to pay for all storage and tow charges as well.
The consequences can be far more serious if you are involved in an accident and do not have insurance. You may have to comply with the financial responsibility laws (SR22, SR22s, SR16) including having to pay for any injuries or damages from the accident out of your own pocket.
It is up to a judge to determine if imprisonment is an option. We hope not!
Being caught driving without car insurance is likely to cost you twice as much as what you would have spent for one year of auto insurance.