Heater lights for about 5 sec and then goe...

One flash on a Ruud furnace indicates failure to ignite. There is a viewing port on the furnace where you can watch the flame. If the ignitor is igniting the burner and then 2 or 3 seconds later, the flame goes out; you most likely have a flame sensor that is dirty or not working. Check to make sure you are getting gas. Check the connections of all wires to the control board. If the sensor is not part of the ignitor, see if you can get to it to clean it with a small wire brush. Make sure there is a good ground connection between the board and furnace frame. If the flame is not traveling all the way to the flame sensor, clean your burners. If the flame burns inside the burners, you may have weak gas pressure.
Good Luck.