While there are many aftermarket parts from reputable companies that are
meant for performance enhancement, many of the generic replacement parts found
at auto parts chain stores are little more than cheap replicas of the original
parts. Manufacturers of these parts make copies that are the same size as the
original, but they are not engineered in the same way and are frequently made of
inferior materials.

Sub-standard replacement parts cover the gamut from basic air filters
to full body panels. Even though an air filter seems fairly trivial, it's
actually very important. If the filter material is too fine, it can clog up with
dirt prematurely, limiting the engine's ability to breathe and make power and
leading to more frequent replacement.
On the other hand, if a filter doesn't capture fine dirt and dust, those
materials can get drawn into the engine where they can scratch and gouge the
metal surfaces that have to slide against each other. Dust particles can also
get past piston rings to contaminate the oil. When that happens, the oil pump
can be damaged and the oil filter can be prematurely plugged. If the oil filter
gets clogged, oil pressure can build up and cause a bypass valve to open,
allowing completely unfiltered oil to circulate.
By skipping the carmaker's genuine replacements for generic parts, an owner
could end up with damage that can cost thousands of dollars to repair.
One set of parts that eventually wears out on every car and needs to be
replaced every few years are the brake pads and rotors. While everyone is
probably aware that pressing on the braking pedal will slow down the car, most
drivers may not be aware of the other ways that the brakes are used on modern
vehicles. Cars and trucks equipped with Electronic Stability Control and
Traction Control can automatically apply the brakes at individual wheels to help
keep the car going where the driver is pointing the steering wheel. These
control systems are calibrated by the engineers based on knowledge of how the
brakes work. If the pads are replaced with aftermarket units that have more or
less friction than the originals, it causes extended stopping distances and
degraded handling.
Insurance companies often try to force body shops to use the least
expensive generic replacement parts possible, but depending on your state's
regulations, you may have the right to request original parts. Replacement
panels that don't meet manufacturer specifications can lead to multiple
problems. The body panels on modern unibody vehicles form part of the vehicle
structure. If a replacement body panel is made from a different grade or
thickness of steel than the factory part, it may not hold up as well if the
vehicle is in a subsequent accident, putting the driver and passengers at
risk.
Since these aftermarket parts are stamped from different dies that
"replicate" the original, they might not be dimensionally correct. This can lead
to degraded fit and finish after a repair and panel alignment issues. Modern
vehicles with steel body panels are galvanized to help prevent rust. The
coatings on replacement parts may not be as complete or as effective, allowing
premature rust to develop. A bad coating can also cause problems with paint
adhesion.
A car or truck is a major financial investment, so it's worth using the
best materials possible to maintain it in proper working order or repair damage.
If you want your vehicle to last, then you should take advantage of all the
engineering and testing that went into its development by using genuine
replacement parts from the original manufacturer. Your vehicle's
manufacturer-trained service and parts advisors can help you make sure you
always have the right parts for the job.