We already know that regular maintenance is the key to your vehicle lasting as long as, or longer than, the automaker intended. For example, Toyota Motor Company makes their vehicles for a lifetime of 400,000 km. Without proper maintenance, however, it would be surprising to see more than 100,000 km from the same exact cars. On the other hand, with regular maintenance, some of these cars regularly top 1,000,000 km. Regular maintenance keeps your vehicle running at its most-efficient, as well as reduces the cost of future repairs.
Sooner or later, even with regular maintenance, every vehicle needs parts replaced and, as with all things, you have some choices. Automobile parts fall into two basic categories, as well as a few sub-categories, each with its own advantages and disadvantages.
Original Equipment Manufacturer (OEM) Parts
Typically, the best parts for your vehicle are the ones that originally came on the vehicle in the first place, that is, Genuine or Original Equipment Manufacturer (OEM) parts. These are the same parts that were used when the car was manufactured. Automakers' strict quality control standards typically keep low-quality parts out of the vehicles they manufacture, so buying OEM parts typically ensures a quality part for the price you pay.
On the plus side, OEM parts are made exactly to fit your year, make, model, and variant of vehicle, so there should be no doubt as to whether the part will perform as designed. Depending on where you buy or who installs it, OEM parts may also come with a limited warranty. On the minus side, OEM parts are typically more expensive than other parts and, depending on brand and where you live, may be difficult to acquire. Closely related to new OEM parts would used OEM parts, which also fall into two categories, those you'd find in a junk yard or remanufactured / rebuilt parts.
Remanufactured or rebuilt parts, typically major components, such as engines, transmissions, or axle shafts, are essentially used parts, but they've been taken apart and put back together by trained technicians to perform almost like new. Worn out parts are replaced and the engine is tested to make sure it works properly. Quality can vary widely, depending on how competent the technicians are, as well as the company's quality control standards. You'll typically pay more for remanufactured parts than for OEM parts, but less than used OEM parts. Remanufactured parts typically come with some limited warranty, as well.
Certain parts simply don't wear out over time, such as a door panel or a passenger seat. Buying a used door from a junkyard, for example, will be far cheaper than buying one new from the dealership, but you may have difficulty finding the right color. Junkyard engines are another good example of a used OEM part that's far cheaper than buying new. Quality can vary widely, however, so be sure to find low-mileage engines, preferably those that have been scrapped because the vehicle it came from was wrecked, and make sure that your mechanic checks it over thoroughly.
One recommendation, however, is to strictly avoid used OEM parts (junkyard) when it comes to safety items, such as brake calipers or master cylinder, and some steering and suspension parts. The lower price just isn't worth the safety of your family.
Unlike OEM parts, aftermarket parts are manufactured and distributed by other companies, some of which are actually OEM suppliers. For example, Tesla Motors, maker of the world-famous Tesla Model S electric vehicle, actually makes very few of its own major components, such as the battery pack, electric motor, and onboard charging system. Many other parts are designed by other companies, specifically for the Tesla Model S, such as the brake calipers and rotors (Brembo), the brake pedal switch (Methode Electronics), and the electronic power steering system (ZF Lenksystems), among many others.
On the other hand, some aftermarket parts manufacturers have no relationship with an automaker. They typically reverse-engineer OEM parts and manufacture new parts to fit the required application. Price is typically lower, but this comes with a caveat, that is, “You get what you pay for.” That being said, quality of aftermarket parts varies widely, from barely functional to better-than-OEM. Also, aftermarket parts may not come with a warranty. Generally speaking, a good aftermarket part typically costs less than the equivalent OEM part, and should come with a warranty. If installed by a competent mechanic, it should provide the right amount of service for a slightly better price.
What Should I Use?
Typically, genuine or OEM parts should be used on cars less than five years old, in order to keep from voiding any warranty that may still be in force. After about five years of age, you should choose between the available OEM and good aftermarket parts that might be available, depending on how long you plan on keeping the car. After about eight years, again depending on how much you plan on keeping your car, you might consider going with cheaper aftermarket parts, or used or remanufactured parts.
Basically, the rule of thumb is, never put more money into a car than it is worth. If the car is worth $500, then it doesn't make a whole lot of sense to put a $1,500 used engine in it, or a $2,500 remanufactured transmission in it. If you have that much money to spend, you'd be better off selling the car for scrap or to the junkyard and buying something a little newer.