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unfortunately the earlier foreman 450's do not have a self diagnosing system. When something goes wrong with the shifting system, then it just goes into a fail safe and doesnt allow you to shift at all electrically. So for these you must do all the checks for the sensors and components that allow these to work. In my experience there have been a few common components on these that tend to go bad. First and foremost the angle sensor. the angle sensor is located on the front center lower part of the engine, it is covered by a metal gaurd. these sensor have been known to even test ok and still be bad. these sensor make up of 90 percent of shifting failures for this atv. next in line would be the connector for the shift buttons. the shift button assembly has a harness that runs down to a large gray connector. these connectors like to take on water, this causes some bad continuities for the computer to read, as well as can cause corrosion as well. again a very common issue with the earlier 450's. i would be sure to check or repalce these items if all your checks are ok. also be sure that the atv will indeed shift ok manually. if not you may have problems internally. the manual shift is a shaft that comes out of the engine on the left side by the riders left heel area. the stock tool kit would contain a wrench that fits the end of it, but a normal wrench can work as well. there is a bit of resistance when using these but it can help you shift into gear in a pinch.