If you have full LED lights (front and rear LED) for the turn signals, sometimes the LED lights do not have enough resistance to heat up the flasher unit

and they will light, but won't flash. You can buy a set of resistors, or a special LED flasher unit, for the LED lights to solve that problem. I've encountered needing the resistors on about 1/2 of the vehicles I've installed full (all 4) turn lights into. If you only have only, say the rear LED lights, and regular incandescent front lights, there doesn't seem to be a problem. The LED resistance issue wouldn't cause the lights to stay on though.

I install at least rear LED lighting in everything I own if I can. Too many hot rods have very poor rear lighting, in this day and age, that is pretty dumb. Gene