subaru hcu

I drive a 2009 Subaru Forester. If I were to lock my keys inside with the engine running, could I disconnect the battery to stop the engine? Would this cause any damage to the car?

As others have noted, this will most likely not stop the engine. Even if it did, you shouldn’t - the alternator’s regulator is not designed to effectively regulate its output current well enough without the battery present to protect the electrical system. If you do this, you are risking damage to nearly every electronic control module in the vehicle - which includes the ECU, TCU (if automatic), ABS HCU, radio, instrument panel, ACU… I can go on. Suffice to say, a *very* expensive repair bill. Also - even if you shut the vehicle off, what is it going to do? It won’t open the doors again, and it won’t turn off the headlights or other electrical accessories (only turning the key to off will do that) so now you have the car still locked up, still with all the accessories turned on, just you aren’t consuming fuel and charging the battery (once you hook it back up) anymore. You’re risking hundreds of dollars in electrical system damage and possibly killing your $100–150 car battery if you hook it back up more than a few hours before the locksmith arrives, vs a few dollars in gas while you wait for the locksmith to arrive. As an aside, this is why the first thing I do upon purchasing a new-to-me car is go to the dealer or Home Depot and have the ignition key duplicated onto several plain flat metal key blanks and one fancy rubber handle one. I keep one at home as a spare for if a friend or my wife needs to use my car, the original I got with the car goes in the folder with the title, one goes on my spare keyring, and another goes in my wallet in case I lock myself out of my car. Five or ten bucks in keys to not ever have to worry about this happening again.