Help with 91 rs trouble codes
 

I got my car running about a week ago but the fan wouldn't come on. So I changed the sensor by the thermostat and the relay. Didn't work so I just grounded the fan at the relay and ran it like that since. My dad was messing with my car and tryed grounding the sensor at the thermostat to turn on the fan. I wasn't there though so idk if he did anything else. I get in my car the next day and it starts and stalls. I try again and get it running but the idle is really bad and the check engine light is on. Code 12 and 15. If anyone has any ideas what the problem could be I would really appriciate an answer and hopefully a solution.

The start of your problems was messing withe the fan control. The fan is control to turn on/off by the ECM. A constant running fan is not the solution. The codes are:
12.....Diagnostic test active>no problem.
15.....Engine coolant temp sensor error, low temp indicated>Temp sensor wired incorrectlly/ or inoperative.

My only suggestion is to correct everything to stock and run it that way. If there is no overheating issues, don't mess with the fan controls.

The first problem is you were playing with the wrong sensor. The sensor that controls the fan is located in the passenger side head between #6 and #8 spark plug. That sensor is probably pooched. Beware that there is another sensor located in the block on that same side. That is the knock sensor, don't get them mixed up. Hook everything back to the way it was stock, put the car on stands, climb under the car, locate the sensor and disconect the wire. Now have someone turn the ignition to "On" but don't start the engine. Now take that single wire and ground it to the block, your fan should come on. If it does you need to change the sensor.

As for the other one, you may have to disconnect the battery for a minute to reset the computer. Hopefully he didn't damage the ECM when he grounded that wire.