The check-engine light came on in my almost brand new car some years ago. I went to the auto parts store, they pulled the code, and told me I needed a new oxygen sensor ... $400! I told DH, who said, "Hmmm ... let's look at the air filter first." Opened up the air filter cover and cups of whole sunflower seeds poured out. Apparently, the mousies were raiding the good stuff out of the bird feeders and hiding it in the air filter box.