Motherboard POST code 0d on ASUS Z790: the fix that works

Quick answer

Clear CMOS (remove battery for 5 minutes or short the CLRTC pins), install RAM only in slots A2 and B2 (second and fourth from CPU), then power on and wait up to 3 minutes for training to complete.

What's actually happening here

ASUS Z790 boards with DDR5 memory are notorious for the 0d Q-Code. That code means the system is stuck in memory training — it's trying to find stable timings for your DDR5 sticks but keeps failing and restarting the loop. I've seen this on Z790-E, Z790 Hero, and Z790-A boards, especially when you first build the system or after changing BIOS settings. The stupidest trigger? Installing RAM in the wrong slots. ASUS explicitly says A2 and B2 for two sticks, but people (including me, once) try A1 and B1 or all four slots and get this error. The underlying cause is DDR5's stricter signal integrity requirements — training fails because the memory controller can't negotiate stable voltages and timings with the DIMMs over a noisy bus.

Fix steps

1. Power off and unplug the PSU. Flip the switch on the back. Wait 30 seconds for residual power to drain.
2. Clear CMOS. The fastest way: short the CLRTC pins with a screwdriver for 10 seconds. Or remove the CR2032 battery for 5 minutes. On some boards (like the Z790 Hero), there's a Clear CMOS button on the rear I/O — press and hold it for 10 seconds with the system powered off.
3. Remove all RAM sticks. Only install two sticks in slots A2 and B2 (second and fourth from the CPU socket). If you have four sticks, don't. DDR5 quad-channel on Z790 is a mess — stick with two.
4. Plug in the PSU, power on, and wait. The board may cycle through codes 15, 53, 55, then 0d again. That's normal. Don't touch anything for 3 minutes. The first DDR5 training on Z790 takes up to 3 minutes — I've timed it.
5. If the system eventually boots (it should), go into BIOS and load default settings. Save and exit. Then shut down, install your other two sticks if you must, but I recommend leaving them out.

Alternative fixes if the main one fails

• Use the MemOK! button — on ASUS Z790 boards that have it (most mid-range and up), press and hold the MemOK! button for 3 seconds while the system is powered off. This forces the board to run a more aggressive memory training routine that may skip past the stuck point.
• Update the BIOS via USB flashback. ASUS released several Z790 BIOS updates in 2023–2024 specifically to fix DDR5 training issues. Grab the latest from ASUS's support site for your exact board model, put it on a FAT32 USB stick in the root directory renamed to the board's flashback filename (e.g., Z790HERO.CAP), plug it into the USB BIOS Flashback port, and press the button on the rear I/O for 3 seconds. The board will flash the new BIOS without needing to POST.
• Swap DIMM positions — remove both sticks, swap their positions (the one in A2 goes to B2, vice versa), reinstall. This can help if one stick is marginally weaker and the memory controller prefers it in a specific channel.
• Test with a single stick in slot A2. If you still get 0d, try just one stick. If it works, the other stick or the dual-channel configuration is the problem. I've seen a single bad DDR5 stick cause this — yes, even brand new Corsair Vengeance or G.Skill Trident Z5 kits.

Prevention tip

After you get a successful boot, go into BIOS and set the memory frequency to the rated XMP/EXPO speed manually (don't rely on auto). Then disable Memory Context Restore if it's enabled — on some Z790 BIOS revisions, enabling it causes 0d on warm boots. Also, if you ever need to clear CMOS again, unplug all USB devices before powering on. I've seen a faulty USB hub cause the board to stall on code 0d during the second training attempt.