Tom, look at this diagram. #28 is listed as a switch and I’m pretty sure it’s oil pressure switch. Figure it may just be stuck from sitting. Might put something against it and tap it lightly and get it to unstick or just replace it. If not that then next would be something in computer box but I’d go to that switch first.
Mercury Outboard Parts by Year (Mercury, Mariner, Mark, Force, Chrysler, Sears, SportJet) Mercury Outboard OEM Parts Diagram for Flywheel & Ignition Coil | Boats.net