OEM (Original Equipment Manufacturer) parts are the exact parts a vehicle was built with. Automakers rarely manufacture their own parts - they contract companies like Bosch, Denso, Valeo, and Gates to build them, then brand them. Aftermarket parts are replacement parts not sourced through the dealership network, ranging from Tier 1 suppliers selling the same part in their own box down to low-quality white-box parts.
In the US, the Magnuson-Moss Warranty Act makes it illegal for a dealer to void a vehicle's warranty simply because an aftermarket part was used. To deny a claim, the dealer must prove that specific aftermarket part caused the failure.
Always use OEM or certified Tier 1 for safety-critical parts: airbags/SRS modules, brake hydraulics (ABS units/calipers), and steering racks and columns, where a failure is catastrophic. High-performance and luxury vehicles (BMW, Mercedes, Porsche) also rely on tight tolerances best matched by OEM or the exact OEM supplier.
Aftermarket is the smart choice for routine maintenance items with no fitment risk: filters (oil, air, cabin) save $15-25 each, spark plugs save $8-12 each since NGK and Denso make about 90 percent of OEM plugs anyway, and wiper blades save $20-40 per set. Over a 10-year ownership period, an all-OEM parts strategy can run $2,000+ versus $800-1,200 for a quality-aftermarket approach.