I'm helping out at my father's auto-repair shop, German Imports, over the holidays, when a 2002 Porsche Carrera comes in with a fueling issue: The owner can't fill it up with gas. We open the hood to diagnose the problem (the gas tank is in the front of a Porsche), only to be greeted with a bewildering mess. The cabin air filter, which supplies the a/c and heating, has been torn to shreds. Bits of paper, trash, and small black pellets are everywhere. After a moment, we spot the root problem cowering under the battery: It's a fist-size rat with the frightened look of a refugee caught at a border crossing.

Animals take shelter and live in automobiles more often than you might think. In the frigid winter your car, insulated from predators and the cold, can be especially alluring to rodents and other critters. And any seasoned mechanic has at least a few stories of faunas in the engine compartment, and not just in beater cars.

"Almost daily we see evidence of some animal, usually rats," says Mitch Montague, a master mechanic at Fletcher Jones Motorcars, a Mercedes-Benz dealership in affluent Newport Beach, Calif., which services about 300 cars per day. "Mainly it's just paw prints in the dust on the top of the engine, empty snail shells, or rat poop. But maybe four or five times a year we actually see a rat jump out of a car."

And depending on the critter, an animal that has made your car its home can do great harm to your automobile or even to you.

As far as your automotive health is concerned, rodents "cause mostly wiring problems," Montague says. "They chew on wires, and then the bare wires touch each other and short things out." Finding the cause of wiring issues is already a pain for mechanics, and rodent problems only add to the difficulty. "They like to chew near their nest or shelter, so it could be in a tight, hidden area where you don't usually think of looking for a problem or a short," he says. In that Porsche at my father's shop, the rat chewed through a wire to a fueling sensor as it ate its way across the car. Montague says he's seen even bigger damage. "I've seen animals chew through the main harness," essentially a car's lifeline of wires. "And then the car won't run at all."

Nests themselves can be an issue. Montague recalls working on a BMW earlier in his career in which rats had transferred more than 25 pounds of dog food and hair into their new-found automotive apartment complex, choking up the engine in the process. For other, larger animals fond of hiding in warm places—such as snakes or opossums—starting the car can spell a surprising death, and create a hell of a cleanup for your mechanic. "I remember we once had an opossum that got stuck between the driveshaft. That was a nasty, stinky mess. There were maggots . . . but I'm sure you don't want to write anything about that," Montague says.

The health risks are a real worry, too, and not just for people with hands under the hood. If the animal is anywhere near your cabin air filter, you could be getting a refreshing breeze of animal dander and excrement every time you turn up the air. "That has to be an ideal way to spread an airborne virus," says Brian Amman, a disease ecologist with the Centers for Disease Control and Prevention. Rats and mice defecate and urinate close to home, and if one's living in or near the filter, "you're practically taking every particle and sending it into the car."

Amman says that many of the worst rodent-borne diseases in the U.S., such as hantavirus, are spread from inhaling excrement particles. Since 1993 there have been only 616 cases of hantavirus, but most were contracted in enclosed spaces with little airflow, such as barns or cabins. Even ignoring hantavirus, "mice and rats are also known to carry sole virus, salmonella, leptospirosis, and other diseases. They're wild animals. Just the dander alone can spur allergies and have negative health impacts," Amman says.

Luckily for Popular Mechanics readers, diagnosing an animal infestation in your car is easier than changing your oil. For smaller critters, look for the telltale signs of droppings and trash, dusty footprints, or nibbled plastic or padding. For larger animals, usually there are obvious signs.

Extricating the animal is another story. For small animals, Amman suggests setting up snap traps, which you should spray down with a 10 percent bleach solution to disinfect before you throw it away with gloves. Live traps are not recommended, he says, as the animal will defecate and possibly bite, increasing the chance of infection. I can personally attest to four ineffective ways of removing a Porsche-dwelling rat: spraying it with a squirt bottle, blasting the undercarriage with an air hose, cursing at it, and gunning the car around the block hoping the varmint flees the g-forces. Neither I nor my softhearted German father wanted to use snap traps, and eventually the rodent abandoned its newly inhospitable home on its own—to spread hantavirus through some Toyota, I imagine.

Prevention is key. If you find an animal under your hood, have had the problem before, or if your car is going to sit for a long time in an area that might be rife with rodents (near alleys or open fields), try to make your car an unalluring animal destination. Park it in the garage, and keep it away from seeds, dog food, trash cans, and spray cheese. All that tasty stuff.

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