What is the ruling on not paying the remaining amount due to a mechanic after he replaced a wrong part in the car, and the problem still persists?

If a worker makes a mistake in assessing the problem and replaces a functional part in your car with another part that was not needed, then he is liable. He must return the old part or buy another part worth its value, and he does not deserve wages for his work; because a common employee guarantees what he damages. You must inform the shop owner of his mistake upon discovering it, and either he re-installs the old part and takes his part without wages, or you pay the difference between the price of the new part and the price of the old part if you keep the new one. These issues require arbitration by experts to estimate price differences.