Abstract: The authorization of diagnosis principals is a critical problem in the remote fault diagnosis of vehicles. Considering the defects of the previous authorization protocol for the remote diagnosis, i.e. PVAUDS, a novel authorization protocol is proposed, named PVAUDS+. In addition to the enforcement on the security properties of PVAUDS, the bidirectional authentication and the freshness of authorization tickets for the diagnosis principals are provided. The resistance of Denial-of-Service (DoS) attack for the trusted third party is also provided. The proposed security targets are achieved through the cost increasing of requests for the resistance of DoS attack, the automatic proof of security properties with the ProVerif tool. The results of quantitative analysis show proposed protocol is practical for use.