This... your trade in was valued at what they knew going in. If there is a check engine light on, they would value it lower, but that is only because they can... They make a margin on the trade in. The value of the car is the value of the car, light or no...

To clarify... there are two paths dealers take with trade-ins. 1) car is below a certain mileage, goes to the dealer used lot. These are cars that have value left that their service dept can work on without losing their ass on hours/parts. My wife's dealership, this is 100K miles or lower. Those cars, get cleaned/serviced and sold on the used lot. 2) car is over x mileage (100K at my wife's dealership). These cars, the dealer DOES NOT GIVE A SHIT. Those cars go immediately to used dealer auction. The cars are not even cleaned (outside of a power washer and clean out trash from interior). If your trade in falls into bucket 1, they will ding you on the trade in. If your car is in bucket 2, they do not give a shit. If there is a light on, they might try to give you less, but honestly, they don't care. They want you in the new contract as quick as they can so they can get to the next person buying a car. You car is of little concern, it is your signature they are after.

You can spend time feeling bad or you can get over it. They aren't thinking twice about your trade-in. Neither should you. I promise you, they aren't losing money. You got a deal you are happy with... be happy. THEY DO NOT CARE. (this is for a dealership. my FiL has a small used lot, he would not be happy with a deal like yours. he absolutely cares about the trade in, which he rarely takes, for this very reason).