Overcrowding episodes in the Emergency Departments (EDs) of the United States and their consequences have received considerable attention by the media and the medical community. One of these consequences is ambulance diversion (AD), which is adopted as a solution to relieve congestion. Current practices on AD decisions are largely dependent on human expertise, thus they tend to be subjective. This paper develops a simulation model for ED to study the impact of AD policies based on one of the following main ED state variables: the number of patients waiting, the number of patients boarding and the number of available beds in the inpatient unit. The objective is to analyze the impact of AD on the ED performance considering two criteria: patient average waiting time and percentage of time spent on diversion. Results show that there exist significant differences based on the variables chosen to design the policy and the criterion to reevaluate the AD status. This insight can be used to assist ED managers in making AD decisions to achieve better quality of healthcare service.