I don't know for sure on those fancy compressor but it looks to me like the pressure switch. It's what turns it on and off. When the pressure drops below 70 psi it closes (on) and when the pressure hits 100 psi it opens (off).
Part number is ARB C035, at least when you use it with an Air Locker. It looks like you have some sort of air chuck next to it so it's possible it's a different on/off pressure values if it's for tires so you may want to verify with ARB.
It's not something you can really fix but it's not super expensive to just replace.
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ARB doesn't make it custom or anything. Any 1/8" NPT pressure switch would work as long as it's the same specs. In fact if you look around you can find similar ones with molded leads or screw terminals that might be more robust in this spot.