help to identify zener diode

Hello all,
I'm still trying to fix my chinese inverter welder. I've chased down some bad zener diodes, but the markings are confusing me. They are marked with c1 and then turn the diode and a 5. I search 5c1 and get results, then I search c15 and get different results. Any idea what this thing is?

maybe it is a 15 volt zener, but not sure.

What does the zener regulate power for?

It is a mini pcb attached on top of the inverter board for my welder. If I'm not mistaken it is the logic control board. It basically consists of a transformer like coil that branches off to 4 sections of the board, each identical. (and each of these 4 sections goes to a group of mosfets on the inverter board) Each of the 4 sections has some capacitors, diodes, and resistors.
Is there any way to test a good zener to determine its value? I think you may be right about it being a 15 volt, just a hunch.

I don't know about testing zener diodes, you can look on the web to find stuff like that.

Get 20v worth of batteries and put a 1k resistor on the positive lead. Now connect the diode between the 1k and negative. Measure the voltage across the diode.