Every year in Germany there are several hundred thousand cases of damage to vehicles caused by marten bites. The culprit is the stone marten, which is quite comfortable around humans. It uses the engine compartment as a shelter - it can hide here from other animals, but also stash its food. If you commute often and park your car overnight in different places, you're particularly at risk. The reason: turf wars between martens trying to claim your car for themselves. Marten bites are a natural behavior most noticeable during mating season.
However, you don't simply have to tolerate marten bites, since they can lead to not only costly damage, but also accidents. If axle boots, coolant hoses, or cables are affected, the vehicle can overheat or be rendered inoperable. Though towing may be free for members of auto clubs, insurance rarely covers the actual damage. After marten damage or the first signs of a busy marten, the engine should be cleaned. Even better, learn about methods of marten defense before it's too late.
An effective, if tedious method of marten defense is to install well-fitted, slotted plastic hoses, through which cables are run. For cables connected to the catalytic converter, these should be made of aluminum, which resists heat. Simpler still is to install wire mesh under the engine or a spike system in the engine compartment. Even more effective are high-voltage and ultrasound devices.
Ultrasound is an effective method of marten defense. These devices create a high frequency not perceivable to humans, but which repel animals. So that the ultrasound method functions effectively, you should look for a sound pressure level of at least 100dB(C) and frequency modulation. Otherwise it won't deter martens, and they'll still use your vehicle as a shelter. Good devices cost around 100 Euros. With this expense you have to keep in mind that the cheaper versions aren't worth the money. Installation should be done such that the emitted sound can be reflected onto the surfaces under the hood. By means of frequency modulation, the tone changes regularly, so that the martens can't simply get accustomed to it. Ideally, it's best to use a combination of ultrasound and high-voltage methods.