I don't have one but what I'd first do is swap the connectors that plugs into the controller box to confirm the problem follows the traffic signal.
As for the solder joints, it seems that it would only take a minute or so to touch-up each joint to rule those out without having to know which one goes to the yellow LED.
Assuming you want to fix it yourself rather than return it, I figure the signal heads themselves are one of those "no user-serviceable parts inside" so not meant to be taken apart. That said, I found the following photo which suggests each head is a can and lid so it comes down to whether you can snap or pry off the lid. Assuming you can get in and it's not a loose internal connection, a replacement yellow LED would be, say, 10 cents - shipping will be far more.
