First let me say that neither myself or any other tech I know will pull one of these units apart to identify the specific part of the failure. We don't get paid for this, it is totally unnecessary for our realm of responsibility and a total waste or our time since we do not have the equipment to bench test part by part. In addition, I really don't care which piece of the puzzle does not fit nor do the customers that hire me to do there installations. About the only individuals that MIGHT be able to answer that are the Raymarine techs that do the repairs when you send your unit back and believe me they don't talk much. my experience and observations, which I comment on here and other forums, is based on my experiences to a large extent on installation of new equipment, recalls for new equipment failures not installation related, and equipment failures of equipment we have not installed. The percentage of Raymarine equipment that fails right out of the box is surprising, but as an installation tech, other than curiosity, I really don't care why it came from the factory and does not work. My experience has been that this is indeed a rarety with Garmin and Simrad and even more so with Furuno. I do install Lowrance and several others and do not encounter the failure rate that I do with Raymarine. And I beleive that members here and at other sites are more interested in that information rather than, was it the circuit board, etc. As to the equipment you have, the displays them selves are the main source of failure although every so often a transducer will fail. With the S1 and most other Raymarine autopilots, the course computer is the weak point and I have replaced three or four computers on a single autopilot. The C90W like all Raymarine displays is very sensitive to voltage, more so than most other units. Once the voltage drops slightly below its desired threshold it begins to do a lot of strange things. The displays are not very user friendly and you must wade through a string of menu items to get where you finally want to be. In addition, the Seatalk is prone to failure at several points and can put your entire system out just when you need it most. Hope this helps. Chuck