Airbus aircraft use diode blocks for indication and control on many ATA systems. Relays, switches, and indication lights will usually have a diode block incorporated with the circuitry. The blocks are easily swapped for T/S purposes. Individual diodes can be checked from the pins with the block removed.
A diode's main purpose is to allow current flow in one direction and not the opposite. An explanation can be found here
The picture above right shows the pin arrangement of the bock. Pin numbers are indentified at the end of each row. The picture below shows a block with the cover removed.
Two blocks (88VD and 89VD) mounted on the back of the fuel pump/valve control panel of an A300-600.
This particular aircraft had a problem with the right center pump not indicating "off" with the switch relaxed (out or off position). It would test with the master dim/test switch. The problem was an open diode (pins 14 & 29) of block "89VD." With diode blocks "88VD" and "89VD" swapped the problem went to the left inboard pump light. After replacement of the bad block, all indications checked normal.
Each block controls numerous functions. A complete panel (system) functional check should be run for all associated systems.