I have a 20 year old gas comfortmaker furnace with a 2 speed fan. Low speed blower fan does not turn off. Is this a relay or wiring problem?

If the fan is being used in the heat mode and will not turn off it is probably the fan and limit control. It is possible a relay could be the problem also BUT I would check the fan and limit first. Sometimes you can tap on the control and see if it turns off.