Open diode reading

The most reliable way to find out whether a diode has failed open or shorted is to test it directly with a multimeter set to diode or ohmmeter mode. An open bypass diode is one of the two main ways a bypass diode fails, and it is one form of Bypass Diode Failure. A bypass diode ends up open when it is missing, poorly connected, damaged by a strong current, or too small to carry continuous current. Open diodes are hard to spot without a direct test, because nothing shows unless the module has a significant mismatch, for example from shading or cracked cells. Open failures are reported less often than short-circuit failures, but they normally go undetected. When a sub-module with an open diode is partly shaded, its IV curve shows a small current without the voltage drop you would expect. With the diode open, current is forced in reverse through cell shunts, which can leave burn marks along cell edges on the backsheet.