DID WE JUST GET STUCK WITH A POS?!?

Bought a car and seller promised in writing to fix certain things 1. Check engine ,TCS, and SRS lights on 2.) when he was cleaning motor/engine with some kind of high pressure hose, the knocker sensor got messed up somehow and 4.)he promised to smog car. Well all he has done is change the knocker sensor but NONE of the other things! And we just dont know if we can get our money back even though we've brought the car home, we havent had any trouble with it YET......CAN WE GET OUR MONEY BACK???? Where do we start?...Any ideas?! any answers will be appreciated THANKS