A diagnostic extender card is plugged into a memory module socket on a personal computer (PC) motherboard. The extender card has a test socket that receives a memory module and an intercepting decoder chip that receives the chip-select (CS) from the motherboard that selects the memory module for access. When CS is activated, the intercepting decoder chip illuminates a visual indicator on the extender card, allowing a user to locate a memory module being accessed. The exact translation or mapping from logical addresses of test programs to physical addresses of the memory modules is not needed, since the visual indicator shows which memory module is really being accessed, regardless of proprietary address mapping by north bridge chips. Operating system memory accesses are filtered out by a counter that counts accesses during a period set by a timer. When the number of accesses exceeds a threshold, the visual indicator is lit.