Can a defendant who pleads guilty to a charge of assault obtain a certificate of probable cause to appeal the conviction?

In general, a defendant who pleads guilty cannot appeal the conviction without obtaining a certificate of probable cause from the trial court. ( 1237.5.) But a certificate is not required for an appeal based on "[g]rounds that arose after entry of the plea and do not affect the plea's validity." (Cal. Rules of Court, rule 8.304(b)(4)(B).) This permits a defendant to appeal alleged errors that occur in postplea hearings to ascertain either (1) the degree of the crime, or (2) the sentence to be imposed. (People v. Ward (1967) 66 Cal.2d 571, 576-577 (Ward).) As to the latter exception, "the critical inquiry is whether a challenge to the sentence is in substance a challenge to the validity of the plea, thus rendering [it] subject to the requirements of section 1237.5." (People v. Panizzon (1996) 13 Cal.4th 68, 76, italics omitted.)