Short of some kind of rube-goldfingers 'halfway takeapart' the
job may be to inspect and clean (or replace) one or both fans.
You may be able to remove bottom of MacBook Pro & see
if there's something lodged in the fan itself. If So, carefully
remove any debris. Also if you have Macs Fan Control app
a D/L runs free, you can select sensors and fan temps there.
Some good info was found among items here; modify for better results:
https://duckduckgo.com/?t=ffab&q=how+inspect+and+clean+fans+in+2012+15-inch+MacBook+Pro&ia=web
If nothing else, a half-hour of technical reading then figure make an
appointment at AASP; for inspection, estimate & repair, is an answer.
Likely too old to wheel into a Genius Bar Reservation, but you never
know until you read ahead toward that direction. An appointment for
Hardware issues, is accessed from in lower section of support page.
An 'Apple authorized service provider' should be able to look into it.
• Apple - Find Locations: (sales, AASP, service, support, etc)
https://locate.apple.com/
Or with good bench skills & extreme attention to details; maybe
you feel up to try DIY; w/ some 'fair to great' instruction videos?
Good luck & happy computing!🌻🐝