Short circuit voltage and no amperage one one of four panels
I've read a number of forum posts with essentially the same title as mine, and got bogged down very quickly in technicalities that are far out of my knowledge set. So here I am in the beginner's section.
I've had a 12v system on my detached garage for around 8 years. Four 120w panels, two Morningstar PWM controllers in parallel charging 410 AH of golf cart batteries. Works fine for me in general.
It has been hard to keep the batteries charged lately, so I checked the panels with a DC clampmeter and found one panel not delivering amperage at all. I disconnected the leads and measured ~20v from the panel. I reattached the problem panel to the controller by itself (it had been attached in parallel with another identical panel) and it still outputs zero amps; its partner panel is putting out 8-9 amps today to charge low battery bank. Each of the other two panels and the second charge controller are about the same (8-9 amps each) The problem panel and its partner are Solarworld 120w 12v panels about 8 years old.
Is it obvious why this panel is not functioning? What should I do to further troubleshoot?