Unexpected memory timings after enabling XMP on ASUS Z790 boards

When this error shows up

You install a fresh DDR5 kit — say 2x32GB G.Skill Trident Z5 Neo 6000MHz CL30 — into an ASUS Z790 board (Z790-E, Z790-F, etc). You enable XMP I or XMP Tweaked in the BIOS, save, reboot. The system POSTs fine, boots into Windows, and you run CPU-Z or HWInfo to check. The DRAM frequency reads 4800MT/s, not 6000. But the BIOS shows XMP enabled. No POST error beep, no black screen, no CMOS clear needed. The board just decided your XMP settings aren't stable and silently fell back.

Root cause

What's actually happening here is the board's memory training algorithm. On Z790 with DDR5, the initial training uses the XMP profile timings. If even one timing fails during the training sequence — doesn't have to be the main CAS latency, could be a subtle sub-timing like tRFC or tWR — the BIOS detects the instability and falls back to the DDR5 JEDEC base spec (usually 4800MT/s at 1.1V). It doesn't notify you via a message or stop at POST. It just silently applies the safe defaults and boots. The reason this happens is ASUS's approach to minimizing boot failures: they'd rather you see a desktop at slow speed than get stuck in a boot loop or a black screen. Most users never notice because they trust the BIOS when it says "XMP Enabled." But it's lying to you.

The fix — force the real XMP timings

You can't trust automatic training. You need to lock in the values manually. Here's the step-by-step that works on every Z790 board I've tested (including Z790-E, Z790-H, and Z790-A). I wrote this for BIOS version 2204 or later.

1. Reset BIOS to defaults. Go to Exit → Load Optimized Defaults. Save and reboot. This clears any ghost training leftovers.
2. Enable XMP I (not XMP Tweaked). XMP Tweaked tries extra tightening that often causes the fallback on its own. Stick to XMP I for now.
3. Go to DRAM Timing Control (AI Tweaker tab). Take a photo of every timing value shown. The board actually populates the fields with the XMP values even though they won't stick during training. Write down tCL, tRCD, tRP, tRAS, tRFC, tRFC2, tRFCSB — all of them.
4. Disable XMP completely. Set AI Overclock Tuner to Manual instead of XMP I.
5. Set DRAM Frequency to the XMP target (e.g., 6000MT/s). This forces the frequency but keeps timings at JEDEC defaults.
6. Manually enter the timings you recorded in step 3. Go back to DRAM Timing Control and type each one in. Don't skip any — if you miss tWR or tRFC2 the board may still fall back.
7. Set DRAM Voltage to the XMP voltage (usually 1.35V for DDR5-6000). Under DRAM Voltage, type it. Also set Memory Controller Voltage (VCCSA) to Auto for now — don't hardcode it unless you know your kit needs more.
8. Disable Memory Fast Boot. Under Boot → Boot Configuration, set Memory Fast Boot to Disabled. This forces full training every boot with your manual values, so the board can't sneak in a fallback.
9. Save and reboot. The first boot will take longer (full training). If it POSTs, check HWInfo or CPU-Z. You should see the XMP frequency and timings.

If it still fails

If the board still falls back to 4800MT/s after manual entry, the issue is likely that your kit physically can't hit those timings at the advertised voltage on your specific board. This happens more with 64GB dual-rank kits on four-DIMM boards. Here's what to try:

• Drop frequency one step — try 5600MT/s with the same timings and voltage. If it works, your kit just can't do 6000 on this board's memory topology.
• Increase DRAM voltage by 0.05V — try 1.40V instead of 1.35V. Keep an eye on temps (stay under 50°C for DDR5).
• Update the BIOS — ASUS released several AGESA updates that improved DDR5 training. If you're on a pre-2024 BIOS, update to the latest stable (not beta).
• Check your DIMM slots — if you're using slots A2 and B2, that's correct. If you're using A1 and B1, you're asking for trouble. Move them.

The real fix is understanding the board's philosophy: it prioritizes boot stability over performance. You have to take the wheel and drive yourself. Once you manually lock those timings, the board respects them and doesn't second-guess you.