3 weeks ago I bought a 2002 Nissan Sentra 1.8 l and ever since I purchased there has been constant problems from where go there was no catalytic converter there was six codes reading all from the Cadillac converter being gone I drove the car about a week then on the freeway doing 95 yes I was very much speeding the car shut off luckily I was at an exit and able to get off the freeway and on the shoulder of the exit to where it took me about an hour to start the car up when I finally started the car to get it all the way off the off ramp it started knocking and there was no RPMs at all so I drove it off the freeway it died at the top of the off ramp and I parked it has not started since there has been oil change because I thought maybe it needed an oil change cuz it was when I did change it it was jet black then I thought it might have been a head gasket so I put the bars head gasket repair in with the oil treatment but haven't been able to run the car to see if that helps I have changed the starter the first time I change the starter I think the person that did it who said he was a mechanic put the wires wrong because the car he was cranking and cranking and cranking about 20 times and their starters being smoking it goes all it's smoking and he kept cranking now today after I've changed the starter back to the original starter cuz I don't think it was the starter I've also changed the crankshaft position sensor I have checked all the wiring I get a battery because the previous battery for some reason went completely dead and no longer usable now all I get is a high pitch buzzing noise or squeaking noise no lights on dash however let me put a side note the previous owner had the car stolen from them so they could steal the Cadillac converter so there is no keys for the outer doors or keys for the ignition of this car I don't know if there's any anti-theft system on the car I have pulled out the ECM because there was two extras in the car when I purchased it so I wanted to see what it possibly could be a reason why there are so many of them however I just need to know what I need to do to fix this stupid car cuz I spent every last dime to get this car and I put about $1,000 in it in the last week