USB Power issue: one port (semi) works, other is dead

On a mid-2012 MacBook Pro 15" Unibody (820-3330 logic board):

This issue seems to have started after using the USB ports to charge R/C aircraft. I've tried SMC and PRAM reset, unplugging the internal battery, but no change.

The front port (closest to the SD slot) seems to be completely dead, and doesn't supply power to devices at all. Haven't checked with a powered hub yet, but I have a feeling that the data lines probably work fine.

The rear port (closest to Magsafe) works, but when I try to charge an iPhone, for example, it only works for a few seconds. I think it's requesting higher charge current, and then it cuts off. Other low-power USB devices seem to work fine on the rear port.

Looking at the schematic, it seems maybe U4600 is responsible for distributing the power to the two ports. I haven't tried any measurements yet, but wondering:

- is there a common failure point in this circuit? or...

- do these symptoms point to a specific component?

- any suggestions on where to start with measurements?

I have a friend who has equipment to handle surface mount soldering, once I can figure out the component(s) in need of replacement. Thanks in advance for any suggestions!