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From the setup guide: If the power light is solid amber - There may be a problem with either the system board or the power supply.

I'd suspect the PSU before the motherboard. See if there's anything helpful here.

BTW: When was last time you replaced the motherboard battery?

I have tested the PSU but it seems ok.

Prior to amber light, the machine will restart a few times before it finally booted up. I have updated the OS to Linux Mint 19.3 and it has been running smoothly until recently.

And when I tried to rectify the problem of restarting - opened up the machine and reseated the memory modules that's when constant amber light came on and since then have tried reseat the ram, changed the CMOS battery, reset the CMOS but all not solving the problem.

Thanks in advance for any help.

Taking multiple attempts to boot sounds like PSU - to me...

How did you test the PSU? It may seem to be ok, but fails under load. You can try the paperclip test, but it's not 100% proof the PSU is ok...

1. Power off and unplug
2. Press/hold power button for ~15 sec
3. Disconnect the main power connector from motherboard (#8 on page 38, here).
4. Open a paperclip and insert one end into the hole corresponding to the green wire, and the other end into a hole for a black wire.
5. Plug PSU into the wall and you should see/hear fans and HDD spin up

If PSU seems ok by paperclip, recheck all cable connections to motherboard at both ends, and reseat all PCI-e cards in their slots.

If none of that helps, strip PC down to bare essentials by disconnecting all drives, except boot drive, removing all PCI-e cards including video (assuming you have add-in video card), removing all RAM modules, except the one in DIMM1 (second RAM slot from the CPU). Then reset BIOS. Close up and connect monitor either onboard HDMI or VGA and see if it boots now...

And if still no go, remove the one remaining RAM module and disconnect the boot drive, and try to boot. You should hear either 2 or 4 beeps directly from the tower. If not, suspect a motherboard problem...