You need to verify with 100% certainty that the mechanic installed the correct oil pressure sensor. Some models are fail-open, others are fail-close. I can't remember which is which, but this issue came up when I had to replace the oil sensor on my M35BC. Installing the wrong one would behave just like "a short in the wiring harness." For this particular item, I would look up the Westerbeke part number for your M35BC engine and get only a genuine Westerbeke part. That's what I did, although I had a very hard time finding someone who had it in stock.
Also, no offense to our Mk1 friends here, but for this particular topic I would be very careful about following any Tech notes for older boats unless you verify that they used the same type of oil pressure switch. Make sure that everyone understands the specifics of your vintage of engine.
Here's a discussion from my own personal experience, with lots of valuable suggestions from others: https://c34.org/bbs/index.php/topic,9214.0.html
Also, no offense to our Mk1 friends here, but for this particular topic I would be very careful about following any Tech notes for older boats unless you verify that they used the same type of oil pressure switch. Make sure that everyone understands the specifics of your vintage of engine.
Here's a discussion from my own personal experience, with lots of valuable suggestions from others: https://c34.org/bbs/index.php/topic,9214.0.html