Car starters engage when you turn the key in the ignition mechanism. Inside the ignition mechanism is a cylinder, which can get stuck. If this happens, then the starter will not disengage as it is supposed to once the engine has started. The starter is not meant to stay engaged after the engine has started, so this causes the starter to burn out.
The starter connects to the battery to operate once the key is turned in the ignition. Sometimes there is a short in the wiring that connects these components, which can cause the starter to receive a current from the battery even if the ignition is turned off. Once again, because the starter is not meant to stay continually engaged, this burns out the starter. In addition, a poorly charged battery can cause starter burn out because the starter will try to operate on less than optimal charge, which can stress the parts of the starter.
Some cars have a solenoid attached to the starter. Specifically, a lever from the solenoid is connected to the starter's clutch and pinion assembly. If the solenoid switch stays engaged, then it continues to pull on the clutch and pinion assembly and will wear out the starter.
A starter is meant to engage only long enough to turn the flywheel and crank the engine. If a driver turns the key in the ignition for longer than this takes, the starter motor is forced to continue operating. Although doing this once or twice won't burn out the starter, doing it repeatedly could do damage.
Although fairly uncommon, some starters have manufacturing defects that cause them to work improperly. The defects can range anywhere from poor connections to bad parts within the starter motor itself. If your starter is replaced but burns out within days, and no one has been starting the vehicle differently than before, you can suspect the burnout was due to a manufacturing problem rather than an ignition, wiring or solenoid issue.
A dirty engine is full of dirt and grime, as well as corrosion from oxidation of metal components. If this dirt and grime gets into the starter, it can act like sandpaper and cause component damage and burnout. The dirt and grime, in addition to the corrosion, can hinder the electrical current from the battery to the starter motor. The low voltage received by the starter stresses it. Incorrect starter fluid levels also stress the starter because the pressure in the system is incorrect. You can prevent these engines through routine maintenance checks and cleaning.
