How to decode the markings on an SMD choke?
Well considering the size, it's probably a 42uH inductor. 42nH is like on the same order the self inductance of a bit of random wire. As in, why would you use that kind of inductor if you can achieve the same inductance with a much smaller, cheaper component? Not to mention you won't even need a component with 42nH in that kind of application.

One way to check is to go on an electronics parts retailer like Digikey and search for 42uH and 42nH inductors and see which one seems more likely. From what I can see, there are a lot more turns than the 42nH inductor I found on digikey, so it's probably 42uH.

Since you're replacing the thing anyways, it might be wise to desolder the inductor and measure it with an LCR meter if you have one, or a simple function generator + resistor combo to get the transfer function. Even an arduino with a LPF will do.