My parents are angry at me because I'm not going to disclose what I know about my car. I'm selling it "as is" and I will tell the owner some things but not all. Is it my duty? I mean I'm going to tell them the check engine light is on but I don't know why.
While it is true that you are selling your car "buyer beware" and you are not a dealer (I assume), you still have to check with the laws of your state. Many states require a private seller to disclose defects. If you live in one of these states and knowingly sell a car that has something wrong with it that you do not disclose, you could be breaking a law and you could be found liable in a court of law. Check with the DMV of your state to know for certain.
I guess it depends on how serious the issues are. If there are things that, if it were reversed (i.e. they were selling and you were buying), would be a dealbreaker, then you really ought to disclose it, in my opinion.
AWOL is correct, most states have lemon laws and require you to disclose all defects so that the buyer can make an informed decision regarding the "as is" purchase. I mean if you knew the transmission was about to go out on a vehicle would you buy it? You might if you knew how to fix it yourself, but probably not. I think you do have a duty to disclose the defects in the vehicle.
I agree with the others, checking the laws of your state is the way to go. Otherwise you can also use common sense. If your car has some serious defects which will compromise the safety of the car, you should let your potential buyers know about it.
I'll call the DMV tomorrow. It looks like I need to contact someone about the lien on my car too. It's been paid since 2006 but I have no paperwork on it!