I have an amber light that flashes four times. The igniter lights for about 5 seconds then the flame shuts off and never heats up the plenum. What do you think that this could be?

Model # Tappan FG6RL 100C-16B

Most likely all you need to do is clean the flame sensor. It is usually a rod in a procelain base & located on the opposite end of the burner rack from the ignitor. Remove the sensor, clean the rod well with a scotch brite pad or steel wool (don't use sandpaper), reinstall, & try it. Let us know how things go.

Somewhere on your heater, usually in the fan case, will be a flash code. They are not universal. For example, 4 on a Goodman is high temp safety which I doubt is yours if it is only running 5 seconds. As Grady says, clean the sensor, NOT the igniter.