

In my opinion the biggest warning sign your car can present is low oil pressure. This is usually displayed as a little red light with a picture of an oil can. If this light comes on while you're driving it means the engine isn't producing enough pressure to properly lubricate it's internal parts. This is the equivalent of having a heart attack. Without engine oil, or oil pressure, the engine will definitely fail and leave you stranded. When the light comes on you should do everything safely in your control to turn that engine off as quickly as possible. This is where seconds matter as the engine can't survive without oil. Whether it's leaked out, or burned off completely, an oil pressure warning light should never be ignored. A good rule of thumb is to check your oil ever time you get gas, or ask the attendant to check it for you. It's cheap insurance against a catastrophic failure.
Another thing you shouldn't ignore are strange sounds or smells. At least once a week someone drives past me with a car that sounds like a squealing banshee. I can't imagine how someone could drive a car like that and ignore the horrible noise being emitted from under the hood. A $9 loose belt will squeal horribly and annoy you, but when it snaps or just falls off you could have big problems on the side of the road. Also, if you start smelling something burning, or smell gasoline while you drive please have the problem investigated. A long time ago I was working at a bank and one of the tellers asked me to look at her vehicle after work because she was smelling gas. I opened the hood and saw that the o-rings around the fuel injectors had failed and raw gasoline was being pumped onto the exhaust manifold. The exhaust maniford gets very hot while you're driving so basically she was driving a bomb. I asked her how long she had smelled gas while driving and she said, "A few months."
My focus here is not to make you feel bad for not maintaining your car. I understand money can be tight and if something isn't broken then don't pump money into it. The exceptions I've listed above do warrant your attention though to make sure you're safe and don't find yourself on a walking tour of our nation's many interstates.