I'm submitting this to the hardware forum, but I'm not 100% convinced it's a hardware problem.

My CPU fan starts when I power on the desktop computer, but then stops at about the time the BIOS screens come to the end and boot-loading begins. If I restart the computer nearly anytime after that (either by hitting reset before I log in and starting again, or by logging in first and then restarting), the CPU fan starts and runs normally until the next time I power-cycle. Essentially, I have to re-boot every time I power on the computer to know the CPU is being properly cooled.

The mother board is a Gigabyte K8NS-ultra 939, the CPU is an AMD Athlon 64 3800+, and the fan itself is, I think, generic and plugged into what is labeled in the user's guide as the CPU-fan connector (distinct from the system fan and the power fan connections). I've checked that and re-checked, so I don't think I've installed the fan incorrectly.

lm-sensors/ksensors shows me what seem to be correct RPMs when the fan is working, and 0 when it's not, which is correct, but not helpful.

I'm running Mandriva 2008.1 Spring, and IIRC, I've experienced this with every Mandriva version since 2007.0.

Has anyone else experienced this problem - annoyance, really?

I would appreciate any direction to begin trouble shooting this, if only to not have to boot the computer twice every time I start it!

Thanks

Have you checked to see if there is some way to turn off any active cooling monitoring in the BIOS so that it just runs the fan at full speed? Another option is to get a fan plug adapter, and hook it directly to your PSU harness. Still another is to check your motherboard to see if there are any other fan connectors that aren't monitored by the BIOS hardware, such as one labeled "case fan" or similar.

Thanks for the tip, Quakeboy. I'll check the BIOS for active cooling (or fan) monitoring.

I've thought about using the case fan connection, but the cord/wire is just too short. I might see if I can find an extension, though.

You could also check for a BIOS update for the motherboard.. might be a known issue that has been fixed.

When you say that this is a cpu fan problem, what part are you saying is a problem? That the behaviour is 'stateful and you find that bad/confusing or that there is a temperature which is wrong?

If the CPU temperature is at a low value, there is no real reason for the fan to run. Whether the temperature is acceptably low is dependant on the ambient temperature, the cpu, the load on the cpu, voltages and clockspeeds and the efficiency of your cooling arrangements. About the only thing there that I have any real clue about is your cpu heatsink, which you describe as generic and so is probably not brilliant (I'd guess it would be an AMD 'packaged with the cpu' one if you don't know anything about it, and they are not brilliant, but they do do the job if requiring a higher fan speed / making more noise than some aftermarket options - so, if that were the case, you would expect at least a moderate fan speed under normal operating conditions - OTOH, if it were a spiffy aftermarket one with heatpipes and all, and in a low-ish tenmperature environment, you may genuinely require zero fan speed for adequate cooling).

So do you have temp data? An independant reading, like a pyrometer reading, would be more reassuring than just the reading from an on-board sensor as sometimes the sensor readings are switched, which could be a cause of problems. Failing that, what does the bios say?

you should check the bios.here fan option could be disable.if this fan is controlled through tharmister, it tharmister could be faulty.in IBM pc i have seen, if cpu temperature goes on normal condition cpu fan would automatically stop.

I would do things in this order, I've had problems like this with a Gigabyte motherboard as well:

1) Make sure the following modules are loaded, type 'lsmod': 'thermal' and 'processor', if they are not listed, 'modprobe' them. If that doesn't fix anything, move on ...
2) Update BIOS, does that fix it ? If not ...
3) There is a BIOS option to disable BIOS control over the CPU fan, this will cause it to run at 100% all the time ... better than the alternative: meltdown

With the Gigabyte board I have, I'm currently implementing #3.

Thanks- all.
Good suggestions. I'll try them.
Salasi, I didn't want to clog my question with too many details, but there's something in the control loop for the fan that's just not right, or consistent. When the fan goes off (immediately after power-up), it stays off, and no amount of waiting will cause it to turn on, no matter how hot the CPU gets. I've monitored this with the case open, and with the case closed using lm-sensors and ksensors.
And if I reboot the PC the fan always comes on and stays on, no matter how cool the CPU is at the moment.

The problem is solved.

Disabling the "Smart" CPU fan control in the BIOS did the trick.

Thanks.