Those Red LEDs next to each memory module light up at Startup.
During the Power-On Self Test, the LEDs on GOOD modules are turned OFF, leaving the LEDS next to BAD modules still lit.
If your Mac will start up, use:
menu > About this Mac > (system Report) > memory...
... to examine this STATIC report:
module slots showing as "Empty" (when you know they are not) have failed the test, and Mac OS will not try to use them until the next Restart, when they will be tested again (and may be brought into service despite the fact they are Bad).
Mac Pro with Xeon processor uses Error Correcting Code RAM:
User Tip: Mac Pro and Error Correcting Code RAM Memory
.