A requisite element to be found guilty of and charged with a California DUI is valid probable cause. An officer has valid probable cause if he observes any Vehicle Code Violation whether it has to do with alcohol impairment or not.
Direct and blatant violations such as weaving, failing to stop at a stop sign, or an expired license, will establish probable cause as required by law. Arresting officers are trained to look for these common violations to pull someone over and issue a citation. More often than not, people are stopped for a simple violation that has nothing to do with drinking. It can be as simple as being pulled over for a broken taillight and the officer will notice symptoms of alcohol such as red watery eyes, slurred speech or odor of alcohol in the car on the person and will initiate a DUI investigation.
In contrast to direct vehicle code violations, in certain situations probable cause is not required. An exception to probable cause is during a welfare check. If you are pulled over at the side of the road, fighting with someone in your car or even if you have a flat tire, a police officer will stop to see if you need help and if you are okay. This is considered a welfare check conducted by the police officer as part of his duty to check upon the welfare of the general public. If the officer conducts a welfare check and notices that you exhibit symptoms of being intoxicated, he will also conduct a DUI investigation. In instances where you are arrested for a DUI during a welfare check, the legal requirement of probable cause is not necessary.