I had an 88auto and a 2.5 from an 02 Impreza auto. Never had that code with with original ECM from auto, a replacement from an auto or a replacement ECM from a manual. My neutral position switch wire to the ECM pin was disconnected. You could try grounding it or disconnected. You could rig up a real neutral switch along the shift rod underneath using a proximity or Hall sender wired to the neutral position switch. Or if it runs OK don't do anything.
