If someone purchases a car from a delearship two years ago (read below?

Whenever I bought it a check engine light came on . Not even a week later I took it back to where I bought it from and the guy told me there was an electrical problem in the engine and it took them over a month to figure out what was wrong with it and two years later (today) I got a letter in the mail saying they are sueing me for the full price of the car that I didn’t even have for 3 days. They are sueing me for over 6 grand .

Additional information
*Purchased in state of Delaware
*No loan was taking out for car
*In house finacing
*Sold me a “lemon law” car
* I paid over a grand in cash for the down payment
* It was a used car
Why can they sue me for a car that I haven’t had in over 2 years ( I just paid off the car I do have and own it now ) and for a car they sold me with problems and they kept?
What are the options ?
*Asking for someone those are their exact words .