I'm troubleshooting a gaming PC with a Ryzen 5 5500, MSI B450M PRO-M2 MAX, RTX 3080, and two matching 16GB Kingston DDR4 modules. Either 16GB stick works normally by itself in RAM slot 1, but neither one will POST when installed alone in slot 2. Interestingly, an 8GB DDR4 stick does work in slot 2, and the system can boot with one 16GB module in slot 1 plus the 8GB module in slot 2. With both 16GB modules installed, there is no display or POST and the motherboard's CPU debug LED remains lit, even after leaving it powered on for about 15 minutes.
I've considered resetting the CMOS, disabling A-XMP, trying default or slower memory settings such as DDR4-2133/2400, setting the rated DRAM voltage manually, updating the BIOS, reseating the CPU, checking for bent Ryzen pins or uneven cooler pressure, testing outside the case, and trying another known-good 2x16GB kit. I'm trying to determine whether this is a memory compatibility issue, a faulty memory channel or motherboard trace, a CPU memory-controller problem, BIOS training failure, or a limitation involving the specific capacity or rank of these modules. Can a memory-channel initialization failure cause the CPU debug LED to light even when the processor itself otherwise works?
4 Answers
The CPU LED can appear when the processor cannot initialize the memory controller, so it doesn’t necessarily prove that the CPU core is dead. Since either 16GB module works in slot 1 but neither works alone in slot 2, the second memory channel or its connection to the CPU is suspicious. The fact that an 8GB module works there makes this less conclusive, though; it could also be a compatibility or rank/density issue with the 16GB modules. Clear the CMOS, run everything at default settings, update to a BIOS supporting the Ryzen 5 5500, and verify the exact Kingston part number against the board’s memory compatibility list.
First confirm the slot and channel behavior carefully. Test each 16GB module by itself in every slot, then test the 8GB module the same way. Inspect the DIMM contacts and the slot for dirt, residue, or damaged contacts, and reseat the modules firmly. If both 16GB sticks fail in the same slot while an 8GB stick succeeds there, the issue is more likely module compatibility, memory density, or training rather than two failed sticks.
Some systems will boot with mixed capacities or an older low-density module but reject a particular 2x16GB configuration. That points toward firmware support or the electrical characteristics of the kit rather than the operating system. Check the exact module model, whether the sticks are single- or dual-rank, and the motherboard’s supported configurations. If a different known-good 2x16GB kit also fails after a BIOS update and CMOS reset, the motherboard or CPU memory controller becomes much more likely than the RAM itself.
Run a proper memory test on each 16GB stick individually, preferably for several passes or overnight, but remember that passing alone does not prove the pair will work together. Also disable A-XMP and manually select a conservative speed such as DDR4-2133 or DDR4-2400. If the system still refuses to POST at safe defaults, check the CPU pins and socket contact, reduce excessive cooler pressure, and test the board outside the case. A damaged CPU pin or channel trace can cause unusual slot-specific behavior.
The mixed 16GB plus 8GB result shows that the board can initialize both channels, but it doesn’t guarantee that it can train with those particular 16GB modules. Try another matched 2x16GB kit if possible before replacing the CPU.