My iMac CPU fan runs on high all the time.

I have done the usual SMC. PRAM resets with no luck. I have tested the fan and the control circuit an I believe they operate OK. I think it may be the LCD temperature sensor that is at fault but would like to prove this before I buy another.

Any ideas how to do this? I will measure the resistances between all pins and post them in the hope someone can test their system and compare measurements.

It's a 4 wire device... is it the same as the Ambient sensor??

One last note, I booted the system w/out RAM and noted that the CPU fan runs at normal low speed. Re doing this with the LCD temp sensor disconnected the CPU fan runs on high....