The issue that arose was whether the single faulty brake light gave the officer reasonable suspicion to stop the car in the first place. Under the North Carolina law, a vehicle was only required to have a single brake light thus, in actuality there was nothing illegal in having a single defective light as long as the other light was working.
In its review of the case, the Court noted that the State had not challenged the interpretation of the statute as requiring only one working brake light. The Court went on to note however that other provisions of the North Carolina Motor Vehicle Code could lead to a reasonable misunderstanding by a law enforcement officer. The Court cited another provision of the North Carolina Motor Vehicle Code that indicated that required all originally equipped lamps be functional.
The Court found that reasonable suspicion can rest on a reasonable mistake of law by the officer. In rejecting Heien's argument that the maxim "ignorance of the law is no excuse" the Court explained that this criminal law concept would apply if Heien were appealing a ticket based on the brake light out, but does not apply to the determination of whether the officer had reasonable suspicion.
The Court concluded that based on the review of several provisions of the North Carolina Motor Vehicle Code, the officer's misunderstanding was reasonable and therefore his actions were based on a reasonable mistake of law. The evidence was properly admitted.