I just noticed that the red led under the lfo button has stopped working. Not a big deal . I bought used but it’s still in the warranty period. Is it an easy fix ? I don’t think it’s worth the price of postage back to Elektron ,if it’s just a cosmetic problem.
Have you tried a reset, already? I know on my MD there is a test function to test the buttons/leds, but I don’t know if that exists on the DT (while holding function and powering off/on).
If you feel comfortable in your ability to take apart the top case to review the solder joints under the led, then that will give you a better idea.
But before going that route though, have you tried the old, trusty method of cursing at the box and/or lightly spanking the area where the light is not functioning? 
If the above don’t work then you can try to reseat the cables inside to see if that does anything before sending it to surgery.
I’m going off of the assumption that it’s either the solder joints under the LED need a re-touch or the led died, both of which are common in electronics, but the machine is so modern that I would assume maybe something may have jiggled out of place during transport/playing 
Yes I tried the test function. Doesn’t light when the rest go red. Thanks for the advice.