CPU fan at high speed due to power supply sensor

The CPU fan on my Imac runs at 3900 RPM. Apple hardware test provides an error code of 4 SNS/1/40000000:TpOP-131.000 which is the temperature sensor on the power supply. I opened the Imac and the single wire to the power supply is connected securely. At this point, the problem could be in the sensor wire ( which disappears into the wiring harness), the power supply sensor - which I assume is built into the power supply, or in the the logic board where the temperature information is processed. I have tried the SMC and PRAM resets with no results. The wiring harness is very difficult to replace, and the power supply is relatively expensive, so I would appreciate it if anyone who has solved this problem could tell me what the solution was. I would really like to repair the Imac and eliminate the loud fan noise.

Hi ee36,
Thanks for your response. I have performed the SMC reset you described many times with no positive results.
The apple repair manual for my Imac says to replace the wiring harness if you have this problem. A daunting procedure for me since you have to almost completely disassemble the Imac. It seems that if the sensor is disconnected (open circuit) the associated fan (CPU fan in this case) will always run at high speed. It seems to me that several failure modes would create this open circuit, and I was hoping that someone could identify how their Imac was fixed so I could hopefully determine the most likely effective repair. I have created an applescript program which I have set up to limit the cpu fan to 1500 RPM and limit the noise, but I would really like to have the security of knowing that the fan could respond to elevated temperatures if necessary. Thanks again.

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