I've had a faulty X2100M2 - it's been claiming for months that it's got a failed fan.
Originally it thought all the fans had failed (although I was somewhat suspicious because the temperatures looked fine). So I replaced them once and that made it happier, but not entirely happy.
So today we tried to replace the faulty fan again. No joy, so we ended up replacing the motherboard. And after fiddling with the cables, we finally persuaded the fault light to go out.
Unfortunately, we were only halfway home. Couldn't get a peep out of the system. Nothing on the serial port, nothing on the SP, couldn't even ping it.
One of the problems was that the replacement motherboard had old firmware, so things like serial port settings were up the creek. I had to play the upgrade game. At least that got me back to sanity, and I could see the system output as it boots.
Whoa there. Couldn't bring the network up. Originally, I had bge0 and bge1; on the new motherboard it decided to call the network interfaces bge2 and bge3. Ok, rename hostname.bge0 and we're back in business, but why on earth was it that hard just to get one of the system fans to work?