Motherboard temp sensor at 123C??

Aux tin sensor?
Moto

Not sure if this is the same, but the Auxiliary temperature reached 44C when running prime95.

HWiNFO64 readings

Is that an uptodate version of Hwm?
try another program like Aida64, just use the trial edition, or Occt
Seems your sensors are playing up there and it would be wise to see what other plrograms are reading temps as, don't forget to check bios readings too, they are usually the best indicator of actual temps although if your sensors are the issue, they would probably read the same temp.
if your Cpu was at 123'c, you would have seen smoke by now though, I don't believe the readings are accurate or correct
Moto

Maybe that 123 C is Fahrenheit? Are using aftermarket CPU cooler?

if you clicked the link Op provided, it definitely reads as celsius, not fahrenheit  
Moto

My CPU fan today started running at full speed 2950 rpm all the time when it never ran before except during boot up.

"Speedfan" and my bios say the CPU temp is 106 deg. C and it stays there all the time.
It shows this temp as soon as booted.

However the heatsink and the processor itself are all cold to the touch, even when the PC has been on for an hour or two.

I suspect that the motherboards CPU temp sensor has gone faulty ( all the other temps are ok according to Speedfan)

I can work around by fitting external speed control to my fan, but will the CPU automatically downgrade in performance if it thinks it is running boiling hot all the time?

Can I fix the temp sensor , or will it be a new motherboard?

If you start your own thread, I'm sure people will help assess your case on its own merit
Moto