I had the same problem when I had a 4th Solo.
It's the code where you have to hold the down arrow right?
The easiest way is to have someone else stand on the arrow while you set in the code.
If you have to do it alone (like I had to) it would seem like it didn't work since you would be standing on the arrow and "stretched" forward to punch in the code.
What I (eventually) did was turn the pad sideways so that I was closer to the control panel and I can solidly stand on the arrow while punching in the codes etc.
Hope this helps.
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If Soujiro started at full speed fighting Kenshin he would have won but nooooo...: