Coil Wiring Harness Replacement

[cbd15178]

Took my boat into the shop last week because of the alarm going off. Computer showed an EST#4 fault. I believe this tells a coil is faulty. The mechanic swapped coils with another one and it didn’t go off again. He also swapped the wiring harness and still could not make it fault again but for some reason he believes that it is the harness. My question is how hard is it to replace the harness and what is the item number? I also have a screenshot I got from VesselView if that helps. I just hate to pay $500 for them to replace it and it not be the correct diagnosis.
Mercury ProXS 250Serial#- 1B826440.

[cbd15178]

I will also add this as a possible scenario....when this first happened, I was just launching one morning. I ran the motor in a bucket when I got home and no alarm. The mechanic ran on the muffs and no alarm. He then put it in the water and alarm went off immediately. He then swapped the coil and wiring harness around while the boat was still in the water and it ran fine. I just put it back in the water after picking it up and it started the alarm immediately. Could something possibly be loose like a connection or something that trailering causes to loosen up?

[Savage]

Sounds like an electrical problem. It shouldn't be difficult to find. Check the harness over real good, or replace it.

[EuropeanAM]

May simply have been the coil harness... did see this occasionally back in that vintage.