I just recently BUILT my computer about a month ago. All 5 fans are running (6 with cpu). My bios and tuner app both say my system fan is running at 0 rpm. What exactly is going on here? Should it be a concern?Not knowing what kind of fan it is, and what kind of motherboard you've GOT complicates matters. So it'd help to know;

Where is your fan getting it's power? The power supply, or your motherboard?
What kind of fan is it, a standard fan or a PWM fan?
Are all the fan's wires connected to its power source?

EDIT:
Basically what your looking for is to see if the Yellow or Green (I think) wire is connected to your motherboard. That's the wire that reports your fan's speed to the motherboard.

If that wire isn't connected and the fan is running, personally I wouldn't say it's a BIG deal. Just means you don't know how fast it's running.endezeichen,Had (actually still have)this very same issue with a Compaq Presario. However it is clear by visual observation that the fan is actually operating.Also the temps are being maintaned well within specs.Contacted HP and they said not to WORRY about it as their installation did not include an ability of the system to detect a fan speed.So it may well be that is the case with the way yours is also setup even though you built it yourself.If it causes other issues you can tell the BIOS to IGNORE the warning. truenorthI have a Striker Extreme motherboard, which supports something like 9 fans, with speed monitoring. Though unfortunately my case's fans came with PSU connections, not the four pin motherboard ones. This means those speeds don't show up, it's not uncommon.