New memory is not detected, or the PC will not boot after fitting it
Almost always seating or slot order. Here is how to prove which.
Safety level: caution · Time: about 20 minutes · Level: intermediate
You fitted memory and now the machine will not start, or starts and ignores it. In our experience this is seating far more often than it is a faulty part.
Applies to
Desktop PCs immediately after a memory change.
Before you start
Power off, unplug, hold the power button five seconds, and discharge yourself on bare case metal before touching anything.
Steps
- Reseat everything, properly. Open both clips, remove each stick, and refit pressing firmly and evenly until the clips close by themselves. If you are closing the clips by hand, the stick is not fully in. This takes more force than feels right.
- Check the slot order against the manual. With two sticks in a four-slot board they usually belong in slots 2 and 4. The wrong pair can prevent booting entirely on some boards, and on others merely halves your memory bandwidth silently.
- Boot with one stick only, in the slot the manual names for single-stick use.
- It boots — add the second. If it then fails, you have isolated either that stick or that slot.
- It does not boot — try the same stick in a different slot, then the other stick.
- Read the diagnostic indicators. A lit DRAM LED confirms it is a memory problem and stops you looking elsewhere.
- Clear the CMOS if it still will not start. Unplug, then either move the CLR_CMOS jumper for a few seconds or remove the coin cell for a minute. This resets the BIOS to defaults, which clears a memory profile the new sticks cannot run. You will need to re-enable XMP or EXPO afterwards.
- If it boots but reports the wrong total, one stick is not seated or is in a slot not populated in that configuration.
About mixing
Mixing old and new sticks of different brands, speeds or timings is the other common cause. It sometimes works, and when it does not there is no fix short of running a matched set. If you mixed, test with only the new pair.
Expected result
The machine boots and reports the full amount. Enable XMP or EXPO in the BIOS to reach the rated speed — without it, memory runs at a slower default, which is not a fault.
If it does not work
A stick that fails in a slot where another stick works is faulty. A slot where every stick fails is a board problem. Either is worth bringing in.
Stop and ask a technician if
- A slot clip has broken
- The board shows a diagnostic code you cannot resolve
- The machine was working before the upgrade and will not start with the original memory refitted
- Anything smells hot
That last one matters: if the original configuration no longer works either, something happened during the change and continuing to power-cycle it is not helping. Bring it in with both sets of memory.
Read this next
- Fit desktop RAM safely — Reseating properly fixes most of these, and there is a right way to do it.
- Work out what RAM your desktop can take — If it still will not post, the module may simply not be one your board accepts.
- It powers on but there is no display — If there is no picture at all rather than a failed boot.
Still stuck?
If anything here does not match what you are seeing, stop rather than force it. Contact us · Book a repair
Last reviewed 2026-08-18.