P0407

There actually is no "sensor B" as you're thinking about it. Such is the problem with generic OBD codes. A low voltage code is probably triggered by an anomalous reading from the EGR valve. Because these aren't cheap to replace, I would try to remove the valve and clean it before running out to buy a new one. Reset the code and see if it returns. The parts can be delicate so don't use a wire brush or anything that may damage the seat of the valve. Check the tube and passages to make sure they are free of blockage as well.