my lenovo G400 is overheating on the board want is the cause

It could be a few things. If under warranty I would contact the manufacturer about it. If it isn't under warranty, and you feel comfortable doing it yourself, first I would open it up and make sure that it is clear of all dust and debris, especially fans and vents. Then make sure the fans are working. Once that is done, close it up and see if it resolves the problem. If it does, great. If not, then see if it is coming from a specific location, the heat. Try running some good temp checking software to see if anything (CPU, GPU, etc.) is overheating. If anything shows as overheating, then you may want to replace that items thermal paste. Should this not fix the issue, then I would look into parts needing replacing. Just be really careful when inside a device.

Again, you always have the option to take it to a tech should you not feel comfortable doing any of the work yourself.