Tell them they need to fix it or buy it back.
I had a new Subaru about 12 years ago that would randomly dump it's coolant and leave me stranded. After it happened 2x I told them I would not accept the car back unless they found the problem and fixed it. A tech spent hours driving it before it happened again, it was the $5 thermostat... without my insisting they had to fix it or buy it back a tech would not have spent hours driving the car.
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