Fan spins at maximum speed by turning MacBook on, then shuts down

Clearly SMC encountered an extreme issue which is likely why the system shut down. I would focus on the power for starters.

What is the color of the MagSafe charger cord LED? Amber, Green or not lit?

After letting it charge does the case feel overly warm around the MagSafe connector?

When did you replace the battery?

The next step would be to disconnect the battery and just try running the system from the charger. Did that also fail?

@arjanj - OK, we have two problems then! First your battery has past its life expectancy! At best with good charging practice you might be able to squeeze out five years, for most four is at best. Just like a car battery the chemistry breaks down over time. But let’s wait on that until we get the system working correctly with the charger.

Inspect the connector surfaces are the pin surfaces clean both sides? Is the plastic area as well clean? If not I would start there. Post a picture of the system’s MagSafe connector straight in so we can see the pin area zoomed in as far as you can go with your camera.

The MagSafe connection has a one wire comm link to the charger with the middle pin. Make sure it’s not damaged. Often I find the knockoff chargers don’t make good contact so you may need a new one. Did you have a friend with a MacBook Pro of the same series? If you do see if you can use his charger to see if it is working on your system.

If the systems connector looks bad you may want to replace it it’s a bit of work but that would be the next step. Here’s the needed part MacBook Pro Unibody 13" and 15" MagSafe DC-In Board and the guide to put it in MacBook Pro 13" Unibody Mid 2012 MagSafe DC-In Board Replacement.

Also do a good inspection of the logic board for any staining or corrosion or burnt components. You might want to use a small soft paint brush to scrub the board and fans of any dust buildup and using a can of can’ed air to blow it out.

Hi @arjanj , Trying another charger would definitely be a great idea, even if the cable or connector itself is not damaged! When power supplies get older, they stop being able to output as much power as they could before, similar to rechargeable batteries! It sounds like the SMC can't detect your battery, so it would entirely be running off the power supply, and if the computer draws what it expects the power supply to be able to handle when it cant, you will definitely experience random shutdowns like this.