When I bought my bike it had no horn.

I bought one of these:
https://www.amazon.com/FIAMM-72112-Freeway-Blaster-Note/dp/B000DINKPQ

Supposed to be 130dB. Plugged in to the horn connectors on the GS, it was weak and horrible. Probably 80dB max. Barely above talking volume level. I tried reversing the leads, worse.

Anyway, I figured maybe it just wasn't getting the voltage it wanted due to wiring, voltage drop, etc. or couldn't draw the current it wanted, etc. So last weekend I wired up a relay and wired the horn straight to the battery via an inline fuse. Glad I spent that $15! Because it had absolutely no effect. Horn is still crummy. Must be a defective horn.

Anyway, now I ordered a $3 generic MC horn from Amazon. Can't be worse than this FIAMM horn.

When you tested it, did you tighten it down fully? As in, did you finish installing it on the bike?

I ask because I just went through horn woes a few months ago. Thought I got two faulty horns but in reality, I was testing them without actually mounting it on the bike. Just holding it in my hand with the wires connected, I thought they were defective but once I actually put it on the bike, @_@ haha I scared my cats!

Yeah it's always been mounted to the bike. I guess it's just defective.

One last thing I thought of to maybe try, is there an adjustment screw on the horn? Or did it come with any instructions?

The one I bought has a screw on the back that's used to adjust the sound/pitch. OEM ones have it too.

If not, then poops ...and good luck with your next new one!