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Legally, under WI law, the purchase of a used car is called an "as is sale." In an as is sale, the courts hold that it is the buyer to beware and that the buyer has the liability for any hidden defects in the car unless the seller gives them a written warranty. Absent a written warranty, the seller has no legal duty to repair or replace the vehicle EXCEPT for very narrow exceptions.
In all as is sales, there are two common law warranties that may apply. First, the implied warranty of merchantability, which is where you need to prove that at the time of the sale the seller knew the car had a material hidden defect that rendered the car unfit for its intended purposes. So, you would need some proof they knew of the defects at the time they sold it to you and then you could sue for breach of the common law warranty of merchantability.
Second, there is an implied warranty of good faith, which is very similar to the implied warranty of merchantability. In the implied warranty of good faith, the buyer has to prove again that at the time of the sale the seller knew there was a hidden defect and concealed it or failed to disclose it when asked. If you again can prove the knowledge of the seller, you can sue under this warranty for the cost of repairs to the vehicle.
If you cannot prove knowledge of the seller, through mechanic's records or car fax records or some means, such as the seller guaranteeing they fully inspected the car and the nature of these defects would have been readily apparent to a reasonably trained mechanic, then you could not succeed in a suit.
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