First thing to double-check is the batteries. How are you testing the health of the batteries? And have you put them through enough cycles to know they're fully charged? (New is never a guarantee of good). Are you sure they're all in the right way? Is there enough tension on the door to hold them in? (Broken battery door tabs are the bane of the more recent YN models).
If it's definitely not an issue with the power supply, then chances are your flash has a bad capacitor (I'm ruling out the flash tube because of the LED faintness). The low-low price had to come from somewhere, and with cheap flashes, there's usually an issue with copy and component consistency. And warranty tends to be the seller replacing a bad unit (which is why you buy Yongnuos from a seller that does this). Service can be hit or miss, as it typically involves shipping to/from China. This is why buying an "expensive" OEM unit makes sense for pros. You aren't subject to a copy lottery.
Replacing the capacitor yourself is not unheard of, but you will need knowledge of how to discharge the capacitor properly so you don't injure/kill yourself just opening up the flash, how to read the capacitance off the old one, a good source for the proper replacement part, and sufficient electronics/soldering skillz.