Thermal sensor reading incorrectly causing the fans to run high

[Z440owner]

The fans in my Z440 will randomly speed up and slow down. I ran a program called Hardware Info and it shows three thermal sensors identified as Motherboard, External 1 and External 2. Sensor External 1 floats around and looks noisy, when it drifts over about 35° C the system fans speed up. I have seen this sensor read as high as 86°C causing all of the fans to run wide open even though the CPU,GPU and memory temps look normal. The other two sensors read normally and are stable. Are these sensors part of the motherboard, if not where are they located? Any help would be greatly appreciated.

[SDH]

Download and install the latest HP Performance Advisor.  It is easy to find via google. and it will update itself after the initial install.  This is the very best method to get an accurate read on all the fan performance and temperatures for the more recent HP workstations (ZX00 and up) and even does a good job with the xw series of workstations.

This software was built with the HP workstation engineer's input and they do know how to probe the motherboard temp sensors and fan speeds correctly.  Other programs are hit and miss...... HWMonitor is seeing less and less over the years so while I still use that I consider the HP Performance Advisor the gold standard.  

[Z440owner]

The HP Performance advisor application is what I first used when trying to indentify the problem, it only shows a single temperature reading (system ambient) and the fan speeds. Refreshing the sensor data will show the fan speeds increasing or decreasing whilst the temperture reading remains the same. I then tried HWiNFO 64 version 6.6 and have found a direct correlation between the external 1 temperature  and the fan speed(s). The graph below shows the temp sensor values vs. time with a 2 second sample interval.Red trace is external 1  The problem is intermittant and the system ran fine for about 3 years at which time the problem started. I'm trying to determine the location of the actual sensor and possibly repair it. If it's integrated into the motherboard I'm thinking I'll have to get a new motherboard.