Help me avoid a possible vehicle scam.?

I just purchased a vehicle from a second hand dealer and I am now doubting it. The following day that I drive off the lot,my check engine light comes on. Took it to autozone and the tech reads a code p0303 which means cylinder 3 is misfiring the day after it went away. I go back to the dealership And they claim its corroded spark plugs. They claimed to have changed that. The whole situation seems wonky. I'm from MA and the dealer is in CT. The car is not registered and has not been inspected yet. Dealer claims that he will not take the car back out of state. First thing in the morning today I'm gonna call my bank to freeze the check before further actions since its only gonna be the third business day of the check clearing. Do I take the vehicle to an official Lexus dealership to get thorough diagnostics or just take legal action since I already paid, do I have any rights? The lemon law of CT states that the dealership is responsible for fixing the vehicle but the dealers confirmation on claiming to fixing the vehicle seems highly biased so they can avoid hefty fees for a possible serious vehicle problem due to the check engine code. Vehicle was priced at 1000 below compared to a first hand dealership. So assuming the check has been cleared and he has my money, what kind of laws are on my side to get my 17k back?