Won't start after changing the ECU

Hi, my name is Devin. I am an auto repair expert with the Car Genius team here at RepairPal. I'm sorry to hear you're having trouble with your Sentra. After replacing the ECU, your car might not start due to programming issues. Most ECUs require proper coding or pairing with your vehicle's immobilizer system to work, so if the new computer hasn't been programmed or matched to the vehicle, it won't allow the engine to start. Additionally, double-check all connections to ensure they're secure, especially grounds and harness connections to the ECU. Since the car was initially misfiring and hesitating, verify that no other sensors (like the crankshaft position sensor) are malfunctioning, as those could contribute to starting issues. If reprogramming the ECU or resolving the immobilizer mismatch doesn't solve the problem, testing the fuel and ignition systems would be the next step. If you need more advice, have any other questions about your vehicle, or would like to get connected to a RepairPal Certified shop near you, we have you covered. Give us a call at 877-913-1554, and we will happily help you out!