Koren Baker has lived in Chicago, Illinois, for most of her life. Her Infiniti gets her to and from work every day without a hitch. Until one morning when her car’s engine light came on. Like any responsible adult would, she took it to her trusted mechanic, Mark Ferjak—an esteemed technician at Berman Infiniti.

He poked around for a little while and immediately discovered the source of her car’s engine light: a hungry rat. It had chewed through her wires, causing electrical issues.

“Wires are chewed right there by a rodent,” he told ABC7. “You can see how they’ll take the insulation, chew it up, make a little nest, and then actually here you can see the excrement. That is a lot, there, and it looks like they were there for a long time.”

He had to replace multiple wires, recalibrate a few things, clean up the mess, and apply some rodent-resistant oil to the new wires to avoid it happening a second time. The repairs cost her around $1,200.

“I was very surprised, because I didn’t know it could be that big in the engine. And we had been driving around with it for that many days,” said Baker.

Rats like cars, but not for the horsepower

Chicago has always had a rat problem—but it gets worse for the humans of the city when temperatures are low. A cold rat can crawl under the hood of a recently run car to warm itself.

While it’s warming its little feet up, it may smell the soy-based oils and lubricants manufacturers use in wiring harnesses. It can also find padding or cloth from the interior and use those materials to build a nest. Before you know it, a rat could be building a family in your engine bay, eating wires, and dismantling the interior.

Ferjak says rodents hit newer car owners the hardest, since factory warranties usually don’t cover the damage.

“Start tracing it down, and then, all of a sudden, you have to tell somebody with a year-old car their car is damaged,” he said. “They have a repair that’s not covered under the warranty, because a rat chewed it the wire.”

There are ways to keep rats from under the hood

Pest control specialists in the area say there are only a few ways to prevent rats from making themselves comfortable in your car’s engine. The first step is to make it less appealing to them by keeping your car clear of food (including crumbs, wrappers, and cleaning up spills).

The second step is to use strong oils, like peppermint, on wiring harnesses as rats find the smell to be too strong for their sensitive noses. Honda makes a rodent-specific tape to wrap wires with infused capsaicin, the compound in peppers that makes them spicy. Drivers can also use cayenne pepper oil.

The third step is to keep your car away from any food sources, like dumpsters. To keep them moving, try not to keep your car parked in the same spot for long periods of time.