If the problems were noted when the warranty still was valid, then one can bring a breach of warranty action. This is because if the problem was not fixed while under warranty, the other party would have failed to comply with the terms of the warranty agreement.
The Lemon Law provision of the warranty act does apply to used vehicles, provided that the dealer manufacturer's warranty is still in effect.
Here is a brochure that discusses the Lemon Law, which is codified at California Civil Code 1793 and is known as the Song-Beverly Act:
To be covered by the lemon law, the car must have been purchased by a dealer, and be under the manufacturer's (not a used car lots warranty) and involve a defect that will substantially impair the use, value, or safety of the vehicle.
If that is not applicable, one can still proceed under the general Breach of warranty action discussed above (but without the Lemon law process). One can sue for damages to have a third party repair the vehicle in a timely fashion (1/2 a year is generally not considered timely) and for economic damages reasonably foreseeable as a result of the breach of warranty (ie rental car costs).