2009 Sportsman 800 wont crank. It was diagnosed as a bad ecu . Changed the ecu and now it wont crank. It does crank with the old original ecu and will start but wont rev up. Have to hold throttle about 1/2 way to keep it running. Fuel pump was changed, tps was re-set. new plugs. eng has good compression.

 

Welcome to the forum. Was the new ECU reflashed or did you just slap it in there? New ECUs always have to be programmed to the vehicle when you install it. Most people have the dealer reflash it. I don't know if that would keep it from cranking or not. I looked online for something to do it yourself and all I could find were programmers for other vehicles. They were $300, so even if they make one for your 800, you may want to have a dealer do it.