What is the test for obtaining a certificate of probable cause to challenge a sentence imposed as part of a plea bargain?

"When the issue on appeal challenges the defendant's sentence following a guilty plea or plea of nolo contendre [sic], the determining factor in deciding whether the issue arose before entry of the plea such that a certificate of probable cause is required is whether the plea agreement specifies a particular sentence or whether it specifies a sentence range. ' "[A] challenge to a negotiated sentence imposed as part of a plea bargain is properly viewed as a challenge to the validity of the plea itself' and thus requires a certificate of probable cause. [Citation.]' [Citations.] In other words, if the defendant agreed to a specific sentence as part of his plea agreement the sentence is an issue that arose before entry of the guilty plea, and in order to challenge that sentence on appeal, the defendant must obtain a certificate of probable cause." (People v. Vargas (2007) 148 Cal.App.4th 644, 651-652.)