Hi, this is Bo, an auto expert on RepairPal's Car Genius team!
If your 2015 Nissan NV3500 won't start after replacing the engine assembly, a few key things could be causing the issue. First, double-check all the electrical connections, particularly grounds, sensors, and the fuel system, as these are often disconnected during an engine swap. A missing or loose ground wire, for example, could prevent the engine from starting.
Also, verify the timing—if the new motor isn't properly aligned (like the camshaft and crankshaft timing), it won't fire up. Another thing: does the immobilizer system recognize the key? Swapping engines can sometimes mess with the vehicle's security system, requiring reprogramming. Lastly, make sure the fuel and spark systems are functioning—confirm the fuel pump primes and that you're getting spark at the plugs.
If all else fails, a diagnostic scan tool will help pinpoint what's missing from the start-up cocktail.
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