[RE-wrenches] panel failure

[RM You]

I have a customer who has 6 siemens 48w panels that are 20 years old (hooked in parallel to a 12v battery). He says that 4 of them have failed in that there is no output unless he bypasses the diodes. However he pulled the diodes and checked them on a diode check and separately on a battery with a light bulb and they appear to be functioning correctly. Any ideas what could be causing this anomaly?

Ron Young