We think we might have an explanation here. A smog certificate is valid for 90 days from the date obtained, for the purpose of vehicle registration renewal or title transfer. It may be, that for some reason, your vehicle's title transfer (if this was a purchase) or registration renewal was not completed/processed and the smog check which you had obtained, at this time, is no longer valid. The question now is, where did the registration process go wrong and why was your vehicle's registration not processed. We'd recommend searching for a canceled check or credit card statement (depending on how you paid) to prove your payment was cashed by the DMV, and then look into why the process was not completed. Was insurance on the vehicle not provided, was there any problem with the paperwork submitted to the DMV. Assuming our assumption is correct and there was a problem with the registration or DMV process, the question is what was the problem and who is responsible. Per California law however, a smog certificate older then 90 days is no longer considered valid.