What is the ruling on repairing a car part that may or may not be repairable, and then the defect reappears after installation? Is a warranty required, with mention of the sayings of the people of knowledge, especially the Hanbalis?

The technician is entitled to his agreed-upon wage after completing the work, even if the part he worked on is not repaired, because the agreement was to install the part and work on it, without stipulating its repair. So, if he performs the work, he is entitled to the wage. Ibn Abi Musa said: "Whoever is hired for a job is entitled to the wage upon fulfilling the work."