07 cobalt, tach quit

bought an o7 cobalt 1700 miles, still under warranty. driving it home, the tach doesn't work, dropped it off at the local dealer, still under warranty. they have had it 5 weeks and can't get the tach fixed. they changed a sensor and went as far as replacing the wire harness from the sensor to the instrument cluster and it still doesn't work. now they are saying someone may have put an 06 motor in it that is different but the only way for them to tell is to remove the valve covers and look at the camshaft but they want a signature from me saying i will approve this and will pay for it. anyone ever heard of this before, if the cam was different and timing was different wouldn't computer and all have to be changed, i can't imagine the tach being the only thing that doesn't work right. this runs just fine. carfax shows it was a rental car, bought at an auction. thanks for any ideas/suggestions

bullshit, march back to that dealership and tell them to fix it, your not paying a dime. Its a warranty, they must pay for the repair work. If you have a problem call GMs main headquarters and file a complaint against the dealership. The main headquarters will call GM and ask them why they are trying to make you pay for warranty work. The dealership will get a mark against it from GM. Trust me, these guys are trying to take your money. Dont pay them a dime, just call GMs main headquarters and file a complaint against the dealership.