Probable cause for a DUI arrest must be based on more than just a belief or mere suspicion that a driver has consumed alcohol and must arise from facts and circumstances that show a probability that a driver is impaired by alcohol or has an unlawful amount of alcohol in their system. For example, if a trooper knows that a driver probably caused an accident, had been crying and had an odor of alcohol then the trooper does not have probable cause to make a DUI arrest without anything more. Â The odor alone is not enough. However, probable cause can arise from the odor of alcohol being present combined with the performance of physical sobriety tests such as walking a straight line, a finger to nose test or a one legged stand test. If the driver fails those it is enough probable cause for an arrest.
Therefore the odor of alcohol must be combined with other factors.
Those factors may also include the defendant’s reckless or dangerous operation of a vehicle, slurred speech, lack of balance or dexterity, flushed face, bloodshot eyes, nervousness, admissions, and poor performance on field sobriety exercises.  If there is an accident and the driver leaves the scene and goes home and the trooper arrives at their home to find them drunk they should not be arrested for DUI as they could have been drinking at home. The only valid arrest would be for leaving the scene of an accident  so long as a witness is available that can identify the driver as the person who left the scene. The arresting officer should testify to the court on the matter of probable cause. Although another officer in the same physical position as the arresting officer who could make the same observations could also testify to the court on the matter of probable cause in the case in question.
Probable cause simply means more probable than not and if so then the officer can make an arrest on that basis.