Aftermarket refers to any product, part, or service that comes from a company other than the original manufacturer. The term shows up most often in the automotive world, where it describes replacement parts, accessories, and upgrades made by third-party companies rather than the automaker. It also has a distinct meaning in finance, where “aftermarket” describes stock trading that happens after a company’s initial public offering or outside regular market hours.
Aftermarket in the Automotive World
When your car needs a new brake rotor, headlight assembly, or exhaust system, you generally have two choices: buy the part made by your vehicle’s manufacturer (called an OEM part, short for original equipment manufacturer) or buy an aftermarket part made by a different company. Aftermarket parts are any parts not sourced from the automaker itself.
The aftermarket auto parts industry is enormous. It covers everything from basic maintenance items like oil filters and brake pads to performance upgrades like cold-air intakes, suspension kits, and custom exhaust systems. Some aftermarket parts are built to the same quality and specifications as the factory original. Some are even manufactured by the same suppliers that made the original part but sold under the supplier’s own brand. Other aftermarket companies reverse-engineer the original part and attempt to improve on its design by working out weaknesses.
Cost and Availability Differences
Aftermarket parts are usually less expensive than OEM parts, though the savings vary by brand and part type. The gap is especially wide for body panels and exterior components, where OEM parts tend to cost roughly 50% more than their aftermarket equivalents, according to the America Property Casualty Insurers Association. OEM parts are typically backed by a one-year warranty from the automaker, and quality is predictable since each part matches what came on the car originally.
Availability is where aftermarket parts have a clear advantage. You can find them at auto parts stores, gas stations, online retailers, and independent mechanics. OEM parts usually need to be ordered through a dealership or an authorized online seller, which can mean longer wait times and fewer purchasing options. If your car is older or less common, aftermarket parts may be the only practical choice since automakers eventually stop producing parts for discontinued models.
How Aftermarket Parts Affect Your Warranty
A common concern is that installing an aftermarket part will void your car’s factory warranty. Federal law says otherwise. The Magnuson-Moss Warranty Act prohibits manufacturers from conditioning a warranty on the use of only their own branded parts or authorized repair services. A dealership or automaker cannot deny a warranty claim simply because you installed an aftermarket air filter or used an independent mechanic for routine maintenance.
There is one important limit to this protection. The manufacturer can deny a specific warranty claim if it can demonstrate that the aftermarket part directly caused the defect or damage. For example, if an aftermarket turbo kit causes engine failure, the automaker could reasonably refuse to cover the engine repair under warranty. But it still could not refuse to cover an unrelated warranty claim, like a faulty window motor, just because you have that aftermarket turbo installed.
Aftermarket in Finance
In the stock market, “aftermarket” has two related meanings. The first refers to the secondary market where a stock trades after its initial public offering (IPO). Once a company sells shares to the public for the first time, all subsequent buying and selling of those shares happens in the aftermarket.
The second, more common usage is “after-hours trading,” sometimes called aftermarket trading. Regular U.S. stock market hours run from 9:30 a.m. to 4:00 p.m. Eastern Time. Trading that occurs before or after those hours is considered after-hours or extended-hours trading. These sessions let investors react to earnings reports, news events, or economic data released outside normal market hours. The specific windows available for after-hours trading vary by brokerage, so you would need to check with your broker to see when extended sessions are offered and whether any additional fees or restrictions apply.
After-hours trading generally has lower volume and wider price swings than regular sessions, which means you might get a less favorable price on a trade compared to placing it during normal hours.
Other Uses of the Term
While cars and stocks are the most common contexts, “aftermarket” applies broadly to any industry where third-party companies sell products designed to work with another company’s original product. Phone cases, laptop chargers, printer ink cartridges, and replacement appliance parts all fall under the aftermarket umbrella. The core idea stays the same: it is a product made by someone other than the original manufacturer, designed to replace, enhance, or accessorize the original.