I searched the forum and found a wiring diagram for the engine coolant temp sensor to the cluster, however, I have replaced the sensor already, and with the sensor plugged in, the temp gauge goes all the way past 280. Even with the car just started first thing in the morning. With a scan tool I can read the coolant temp at around 150 after driving around for a while, however when I use a handheld temp gun, it reads around 195 at the thermostat. Also, the fans never kick on with the sensor plugged in, because the PCM only sees around 150 degrees. I have been driving with the sensor unplugged because the fans will stay on and I feel safer driving this way, and the temp gauge reads below 100 (-40 on the scanner). My question is, does this sound like a PCM failure or a gauge cluster failure? Thank you for your time. Love the forum so far.

What vehicle?
Either way. Are you sure the sensor is not shorted. NIB don 't mean it'* good.
Unplugged it should read -40. The wiring is not shorted or it would read full hot all the time. The sensor is where I would look.

98 Bonneville non supercharged. I am thinking about replacing the sensor again, since the previous owner had it replaced and I dont know what brand it is. I'll try to find an AC Delco. I agree with you that the wiring seems to be ok.