the coolant temp switch, just unclips, you pull the clip out of the side of the coolant switch housing under the switch, and pull the sender out, careful not to lose the seal off the switch. remove the seal, fit to the new switch, add a little vaseline to the seal, and refit. if the switch has 4 wires going to it, the gauge can work even if the engine doesn't read it right, as it is a dual switch. if its has 2 wires going to it, you need to replace the blue switch, as this one will be for the engine. before you go ahead and replace the immobilizer reader coil, its worth checking the fault memorys of the engine and immobilizer 1st. if the imobilizer is the cause of your problem, then the engine fault memory will have stored in it 'engine control unit blocked.' if its clear of this fault, then its unlikely that the immobilizer is the cause of your issue. best regards, dan.