Question win 10 pro wont boot unless config changed

Aug 5, 2019
Trying to diagnose a problem with my refurbished Dell 9020 tower: Windows 10 pro, 4th gen I5 processor, 16gb RAM, 250gb SSD (boot disk) 1tb HHD (storage), 1650 4gb graphics card, and it's a bit of a strange one. The machine fails to boot up from a cold start (ie after being powered off). The fans are running and the power light is white (correct), but no signs of life apart from a brief flash of disk light every 3 seconds or so & nothing on the screen. No beeps or power button error light.

I suspected memory, the machine has 4 x 4Mb dimms and I found that by removing & re-inserting any of the memory it will then start up, after 'memory changed press f1' and then runs normally.

A warm restart (ie restart selected from start menu) works perfectly OK, but after powering down completely, it won't start again. Guess it does some checking on a cold start that it doesn't on a warm start.

Tests done:

Shuffled memory around, started up OK every time (as expected after removing/installing memory).

Run full ePSA (F12 on startup) diagnostics with all memory installed - no issues found.

Checked BIOS error log, nothing.

Took out all memory, then tried each spearately in slot 1 on the board and booted up after each. All worked perfectly, so assume they are basically OK on their own.

Tried one memory stick in each of the 4 slots, booting up after each change, all worked perfectly, so hopefully the motherboard is ok.

Thinking maybe it's the combination of memory DIMMs the machine doesn't like. I did notice that one of the DIMMs is a different speed to the other three, they are all 12800U, the other one is 10600U (the only one branded Dell so maybe the original). Having said that, if I was to remove only 1 dimm and re-start, them computer works fine (after F1 to confirm config change), regardless of which dimm I remove.

It doesn't matter what I change with the memory, as long as it's enough to trigger 'config changed press f1 to continue', all is well.... until I switch it off and then try to restart.

Could this possibly be the CMOS battery? Running out of ideas as to what the cause might be.