In what circumstances will a defendant be able to challenge his conviction and sentence if he failed to obtain a certificate of probable cause?

Likewise, in a case entitled People v. Vargas (2007) 148 Cal.App.4th 644, 55 Cal.Rptr.3d 837, the defendant did not obtain a certificate of probable cause, and we addressed and rejected the defendant's argument that the trial court violated and unilaterally modified the terms of his plea agreement. ( Id. at pp. 651-652, 55 Cal.Rptr.3d 837.) However, we declined to address the defendant's remaining claims in the absence of a certificate of probable cause, because they attacked the validity of his plea. ( Id. at p. 652, 55 Cal.Rptr.3d 837.) These claims included challenges to the specific sentence negotiated as an integral part of the plea agreement and an argument that the sentence violated the prohibition against multiple punishments in section 654. (148 Cal.App.4th at p. 652, 55 Cal.Rptr.3d 837.)