Test for item in output slot of furnace

There are two types of furnaces:

1. furnace (furnace turned off)
2. lit_furnace (Furnace when burning something)

The data Value has to be -1 or the actual rotation of the furnace (-1 means it does not matter).

So change your command to:

/testforblock x y z minecraft:lit_furnace -1

The output slot is 2b and iron is called iron_ingot:

/testforblock x y z minecraft:furnace -1 {Items:[{Slot:2b,id:"minecraft:iron_ingot"}]}
/testforblock x y z minecraft:lit_furnace -1 {Items:[{Slot:2b,id:"minecraft:iron_ingot"}]}

You would have to test if one of these testfors was successful (basically do your redstone 2 times, one for each command).

Source.