When NHTSA receives enough complaints or evidence suggesting a potential safety defect in a vehicle, it opens a formal investigation. The process typically begins with a Preliminary Evaluation (PE) to assess whether a defect trend exists, followed by an Engineering Analysis (EA) if the evidence warrants deeper examination. Investigations can result in voluntary recalls by the manufacturer, mandatory recall orders, or closure if no defect is confirmed.

An "Open" investigation means NHTSA is actively reviewing the potential safety defect, engineers are collecting data, analyzing complaints, and may be testing vehicles. A "Closed" investigation means the review is complete. Closure can mean the manufacturer issued a recall, NHTSA determined no defect exists, or the issue was resolved through other means such as a technical service bulletin or design change in newer models.

Roughly half of NHTSA investigations result in a safety recall. The rest are closed without a recall, either because the defect couldn't be confirmed, the failure rate was too low to warrant a recall, or the manufacturer addressed the issue voluntarily through other channels. Investigations involving higher injury or fatality counts, or those affecting a large number of vehicles, are more likely to result in recalls.