This happened to me early this season as I was setting the anchor... scared the heck out of us until I realized one drive was not shifting to nuteral.
My issue was a retaining clip at the drive had come loose... 1/2 hour swim in cold water solved it.
Your issue will be at your shifter or drive. Not sure what shifter you have so can't comment on what to look for.
At the drive your shift cable comes through the transom and into the back of the drive on the starboard side. The outter casing of the cable does not enter the drive and gets secured by a retaining clip. If this clip is not in place the cable will not work properly. If you feel around and find this cable, it should not move when you try to wiggle it.
Once the cable passes into the drive it connects to the shift lever with a cotter pin. If it's there and connected it should work. You can grab the shift lever and move in and out of gear at the drive to confirm no issues.
To see the shift lever at the drive remove the large plastic cap on the drive.... the one with the SX decal and secured by 3 screws.
Good luck.
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2004 - 288 Vista
Twin - 5.0 Volvo Penta GXi DP
