A couple of my Nitecore Tubes worked well for a while.
Now when I switch on to High the light flashes, works OK on the low setting.
I have other Tubes that are OK - for now.
Anyone have any ideas what can be wrong & is this fixable ?
my first tube did exactly the same after a few weeks of use suddenly flashes on high but works ok on normal, i think a defective battery or bad solder job?, my other one works ok so i guess some quality issues on some… i think since this tube is not exactly a super expensive light keychain certain quality flaws exist 
Im sure u can fix this if u have some skills but i cant be bothered to do that since i dont know exactly whats wrong, maybe someone in here can help?
I have looked on the Web & this seems to be a common problem.
In my opinion a failure rate of 2 out of the 4 Tubes I have mean only one thing.
A rubbish product that I will not buy again.
Although they are fairly cheap from various Chinese dealers its not worth the effort of trying to claim anything under warranty.
I had the same experience. Seller was good enough to send a replacement. I opened the bad one. The problem is that the LED has failed. It works in low mode but when the voltage goes up, it starts to flash and finally turns off. I replaced the LED and all is fine. I believe they are driving these LEDs beyond their safe range. The voltage at the LED at full brightness is the battery voltage which can be as high as 4.2V. White LEDs are generally designed for maximum 3.4V. Sure, they are very bright at 4.2V but if they stay on for a while, they will burn out. The other problem with these lights appears to be the switch which can turn on if it’s in a pocket. I can see the light turning itself on in a pocket and burning itself out without you even knowing. Nitecore should really throttle the voltage a bit on these.
Like many other Nitecore products, this light seems to combine a good concept with crappy execution.
I bought two long ago. One still works correctly the other flashes incorrectly (50/50). The TIPs replaced them…