The Check Engine light came

The Check Engine light came on in the new car the other night, as Holly was coming home from work. I took it in to the Toyota dealership, as the car is under warranty, and they told me that the computer said my gas cap was faulty, so they replaced it. How much money would it cost to add an LCD display to the car that gave me a 4 digit error code, telling me what was wrong with my car? Of course then the dealership wouldn't be able to charge people for these stupid things. I'm glad my car is under warranty until 2007.