You can sue the dealer for fraud for selling the car to you without a catalytic converter.  Depending on what information is included on the certificate, it may provide additional evidence of fraud.  All cars manufactured since 1975 have a catalytic converter.Without a catalytic converter, your car won't be able to pass the smog test in CA.  If the certificate indicates that it passed the smog test, that could provide further evidence of fraud.Your damages (the amount you are seeking to recover in your lawsuit) would be either the benefit of the bargain or your out-of-pocket-loss.  If you decide to have the car repaired, the dealer would be liable for the cost of repairs.  If you decide you don't want to keep the car due to the dealer's fraud, your damages would be your out-of-pocket loss (the price you paid for the car) or benefit of the bargain.  Benefit of the bargain would be the difference between what you paid for the car and what you should have paid for the car given its condition (no catalytic converter, etc.).Depending on the language in the express warranty, you may have another cause of action (claim) in your lawsuit for breach of express warranty.