Check the sensor for signs of wear/possibility of being knocked about.

Get your bf to flush the system and use Subaru coolant with distilled water. There are cheaper coolants on the market but for what it could cost you in the long run its not worth it.

Also see if you can see any signs of coolant leaking on the hoses or radiator itself, helping you to trace it back to the source (if this is the issue).

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