It would help if you said what fan, and give us the model number and manufacturer of the board. Most boards today don't have a fan on the board, and if you are talking about a fan header for a case fan or the cpu fan, well, for a case fan, just connect it to the power supply with an adapter, for the cpu fan, there is no work around, as the rpm sensor is tied to the bios, and will keep the board from completing post to protect the board. Chipset fans, it's usually the fan that goes bad, and it will need to be replaced.