My Dual 2.0GHz PowerMac G5 had one of the CPU's go faulty around a year ago, I thought it was the end of the line however managed by luck to pick up a matching pair of G5 CPU's for it, the serial numbers of the new CPU's were also from the same batch as the ones that originally were in the machine which was even better luck.

Anyhow, after replacing the thermal paste, checking things over and replacing the CPU's the machine sprung to life again with the usual symptom of the fans going a bit crazy. Not a problem I thought, download ASD 2.5.8 and burn that to a CD I thought.

After running Thermal Calibration most of the fans settled down with the exception of the rear exhaust fans, the top one (CPU 0) for some reason seems perpetually stuck at 100%, thermal Calibration completes successfully but the fan remains at 100%.

I bought the PowerMac G5 back in 2005 brand new as a custom build to order configuration, want to keep it alive as long as possible as I've grown attached to the machine, it was in daily use up to around 2 years ago just before the CPU failed.

Any ideas on this one?

I wonder if the thermal sensor in those areas is failing. Unplug it & recalibrate - see if the fan behaves.

I did think that however running the hardware tests on the ASD 2.5.8 CD for the sensors and they all pass.

I replaced the rear exhaust fan assembly as well to rule that out and get the same behaviour with the new fans.

well, lots of pmg5 owners use the software and if you replaced the fan, that rules those bits out as likely points of failure which leaves the thermal sensor. I'd unplug the thing, reboot and see if mach10 mega fan speed goes away.

Thanks again, further digging and I can see that Apple Service Diagnostic no longer detects the CPU1 Exhaust fan as if it isn't there to test, the other 6 fans are showing. Running any tests involving the memory now also cause the machine to unexpectedly shutdown so I'm thinking unfortunately the logic board may be on its way out.

Apart from the fans spinning full the machine does boot and run apps fine which is the odd thing.

Ahh man that is a drag. Could it be the version of software? Maybe try the earlier 2.5.7 (I think thats right lol) or a later revision? Its worth a shot.

Tried ASD version 2.5.7 with the same results, it doesn't detect CPU1 Exhaust Fan as a component to test and get the same results trying to run the tests.