iMac G5 Temp sensor problem

[squelch41]

Hi,
I've got an original 17" imac G5 and seem to have the opposite problem to everyone else with it!
The fans dont spin up fast enough under load and, eventually the system will suddenly enter sleep.
This all seems like overheating.

If use software to find the CPU T-Diode temperature, it reports 1.4C which is obviously wrong.
On the Apple Hardware Test, I get a error of 2STH/1/3:CPU-Tdiode

At times, the system does read the temperature - it will sometimes show use usual 50-73Cish and the fans ramp up and down accordingly.

I assume there is a faulty temperature sensor but I don't know where the CPU-Tdiode sensor or traces are -- are they internal to the CPU or is it on the logic board itself?

The capacitors on the logic board all look good and they system runs fine otherwise.
I've tried resetting the SMU but that made no difference.
The HDD temp sensor works

Any pointers would be great.
Would rather try and repair if I can rather than replace logic board.

 

[GiGaBiTe]

If I were to guess, the thermal diode is on the G5 die, or directly under it on the logic board.

You could try using cold spray, or an inverted air duster around the CPU while its running to see if you can get any temperature changes, which would narrow down where the diode is.

 

[ClassicHasClass]

It is indeed part of the processor die. If it's defective, you might be able to try messing with the fan control software, such as making the RPM constant for all temperatures; see something like https://discussions.apple.com/thread/3656062 . There isn't an equivalent of G4 Fan Control for the G5 that I'm aware of.