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First, a few basic clues; boat is a 2013 R5 with a VP 380 CB.

Second, while replacing the fuel pump (story on whining fuel pump thread), I had to move the coolant tank to get access to the fuel pump. In doing so, the sensor in the picture came loose from the coolant tank, mounted on the front driver side of the engine.

Third, and this is where I'm stumped. This "sensor" clips on the underside of the coolant tank and the "probe?" end of it goes into a recess in the coolant tank, but does not touch the coolant. The recess is just that, an indentation in the coolant tank for this sensor (or whatever the hell it is!) to sit in. Since there's no contact with coolant, I can't imagine it's for temp or level.

I tried to follow the wires, but they disappear into the abyss of the wiring harness deep in the belly of the beast!

What do you think?

Do you see it anywhere on these pictures. So I guess what is your question? Are you getting a warning. Why did you remove it?

Look at the first picture and you can barely see a blue connector right above the red wire on the fuel pump. That's the connector for this sensor, or whatever it is. It unclipped from the bottom of the tank when I moved the tank out of the way. If you look at my picture, the round part clips into the bottom of the tank. It goes into an indentation in the bottom of the tank. Can't imagine it's smart enough to sense level or temp without being in contact with the coolant. The indentation is large enough for me to stick my pinky finger in it, almost all the way.

I plugged it back in during reassembly and all seems well. Just wondering what the heck it is and what it does.

Not sure if could be a air temp sensor. Hard to say. I guess go to the dealer and go peek at another one. Or one of the guys on here can look also. Without actually seeing it in person I can't tell.

Some sort of coolant overfull or overflow sensor maybe? Difficult to imagine what it might be from the description.

It's on the coolant tank schematic on Marine Parts Express listed as a sensor for $15. I'm guessing it has to be ambient air temp related.

I agree. I can't find it on the Volvo parts though. They don't have a number by it and it's not shown on the harness picture either. But I'm trying through my phone.