I used to build PC's with Pentium 1's and 2's. So I tried to repair my HP m8517c. It had a defective Coolermaster heatsink. Two of the plactic screw mounts broke off. I replaced it with a Coolermaster Hyper 212 Plus with a larger 120mm fan. I'm getting a CPU fan failure message when I boot. I stuck the old 80mm fan in with paperclips and it works fine. What gives with the 120mm fan? Is my 300 watt power supply to small for that fan?

Nothing to do with the power supply, how did you hook up the fan to the available motherboard pins on the fan power mount?

It was connected to 4 pin connector on MB for the CPU fan. The case fan power is a 3 pin connector. I've tried 2 new 120mm fans and got the same error. The 2 new fans are different brands. One is the Coolermaster that came with the heat sink the other is a Rocketfish.

it could be theres an rpm/fan voltage setting in the bios thats been turned down too much to reduce noise, but if its turned down too much fan wont even start