The ECTS is the Engine Coolant Temperature sensor. I don't know which engine you have but on the SL1 it is located on the side of the engine block, on the drivers side of the car. It is a small sensor that screws into the block and has two wires coming out of the end. You'll need to unclip the plastic connecter taking the two wires off, and use a deep socket to take the sensor out. Have the new one ready, and wrap the threads with teflon tape for a good seal. As soon as you take the sensor out the coolant will start to dribble out, so have the new one ready to screw in right away. The sensor isn't very expensive at all.