Where is the cylinder head sensor located in 300zx?

I think you're talking about the cylinder head temperature sensor. It's well hidden. It's located on the front toward the bottom of the cylinder head on the driver's side. When standing in front of the car it's just above and to right of the water pump. The easiest way to locate it is to find the two wire cable that goes to it and follow it down behind the timing belt cover, past the coolant pipes and almost behind the other side of the timing belt cover. The two wire cable will have a connector near the number one spark plug. I expect that your computer is flashing the 1 red 3 green code. Most of the time the problem is the connection between the two wire cable and the connector. On the two I saw, the connection was very corroded. It took a lot of cleaning and some bending of the contacts on the connector before I got a reliable connection. Once you find where the two wire cable ends then take the connector off the sensor and try to clean both sides. Be careful taking off the connector - it's probably brittle by now and the spring retainer clip easily goes flying to who knows where. I took a long handled screwdriver and partially pried up the spring retainer. I then clipped on a hemostat to the clip and finished removing the clip. The connector then comes off easily. Bob PS - you'll have about 2,000 Ohms of impedance across the wire harness when everything is good. The sensor is a thermistor so the resistance will depend on how warm the engine is. Usually it'll be 1500-2500 Ohms when the engine is cool enough to work on.