The "L" lock and the button press do the same thing. So if you press the "L" key, the "S" thingie should also show "locked", so changes from "red" to "green". If you then press either one another time, they will both "Unlock" at the same time, the "S" turning red again. If that is not happening, then you might have "mod soup" with a partially activated mod, which can happen in two ways: 1. Having installed the game in any of the "Program Files..." folders, including if using the Steam game 2. Failing to empty the Save folder. Either way, and you can end up with odd / strange behavior from the game. You can also encounter another form of "strange" if you are in the habit of "shelling-out" to the Windows desktop while playing the game. You might encounter graphics and/or audio issues if doing that.
The "Lock" itself centers the targeted ship. If you then press the PK button, the TDC will begin tracking the ship, and will continue to do so even after lowering the periscope. So get a lock from a distance, and then using the PK (Position Keeper) will allow you to submerge potentially beneath a thermal layer as you approach your quarry 'stealthily', maybe even on Silent Running". You do not need either the lock or the PK to shoot though. Your periscope will not track the target when you cannot see it, so when you come back up to PD to shoot, you'll want to use your sonar so that you know where to initially look. Open your torpedo tubes, and be sure your PK is "off", then bring the scope up, point it where you want to shoot. If the periscope does NOT track the change and update the TDC, you can hit the PK to bring all the data back into the TDC, then shoot. The torpedo should go about where your cross-hair is when the shot is made - maybe...
The "green" arrow just means that you have a good solution, and the "yellow" means an OK solution. You might be too far away, or at a large attach angle, or whatever, if it's yellow. Red means that the TDC (in auto-targeting) cannot properly track the target. It would be tough to say where your shot might go. One of the main causes of that is using an incorrect tube for where you are looking. If the target is behind you, say at 170 degrees, make certain you are using the after tubes... There are a few "Targeting" missions in the Sub School section that can be used for practice. The enemy does shoot back though in SH4...
