AMS1117 3.3V regulator outputs 4.2V or 3.3V depent on source of 5V input
Sounds like it is defective, assuming you've connected and measured it correctly.
The part you mention is a self-resetting fuse (maybe 0.5A rating) and should have minimal voltage drop in normal operation.
Check that there is not an open solder joint on the AMS1117 leads (esp. the left-hand one).