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It seems every time I update the Durability on the car, the part never fails. All durability cost 500 RP. If a part fails in Powertrain, Aero or Chassis, it cost at least 500 RP to fix and another week before it successfully goes on the car. I've noticed if I save 500 RP and do a durability upgrade the same week an Engine, Aero or Chassis part is being upgraded on the car then the part never fails because we only get one cutscene with our engineer. Doing it this way, I guarantee any big upgrades that might cost 1500-2000 RP will not fail. Try it and see, just save an extra 500 RP and you can upgrade the car without failures and upgrade durability at the same time.