BIOS shows wrong motherboard temperatures

Question

I have recently built a Mini ITX system based on a Pentium 4 CPU (P4 HT 631).The CPU was bought off Ebay for 5$ and the motherboard was salvaged from a local recycling center. The system works fine, it boots in Windows, i can browse the internet, watch videos ,etc.. .The only problem is that it runs quite hot (expected because of the P4).

The system:

- Pentium 4 HT 631
- Commel LV-672 system board
- 512 MB DDR2 400 SDRAM

So i wanted to see how hot it gets when idle and i stuck my k-type multimeter sensor inside the heatsink.The results:

• the multimeter shows 49°C (i can feel on my hand how hot the heatsink is)

• the BIOS shows only 27°C and even worse, the system temp is 19°C ! (while the room temperature is 25.2°C)

UPDATE: I got to test the Pentium 4 CPU with another motherboard and it shows 48°C in the BIOS, as well as on my multimeter. Right now my guess is that the motherboard has a problem.

UPDATE 2: After a couple of hours running, the temperatures shown in the BIOS grew gradually. Right now, the CPU is at 37°C (multimeter is still at 47°C as it should be) and the system temp is at 27°C. This is weird. Also, i am now using a standard Intel cooler.

What could be wrong ?

Answer 1

What your seeing is usual, for some boards, especially of that time/age. The "CPU temp" in the bios is aquired off a sencor that is on the board itself under the cpu , not in the cpu.
A "core temp" is the only temperature that is aquired off a sencor that is inside the chip of the cpu, and where that sencor is can even make minor differences too.

Both sencors are an analog resistive temperature probe that is analised for the resistive values and digitally converted, and have always been subject to errors in calibration and part differences.

The CPU is not idle: When your in the bios, the HLT is not active, and the cpu itself is not actually fully idle, it takes software to implement the standby states, then and now.

So yes it is off, and it can be off more or less, more often on cheaper boards, and smaller boards are often the cheaper ones. I do not yet see a problem, it has not reached temperatures that would cause a failure , even P4s of the 775 socket will throttle back on normal overheats.

I would advise you to continue your efforts, stay aware of your cooling as you already are, and calibrate any software before believing it represents reality. Run some hard benchmark tests with your external thermal probe, to insure your cooling is sufficient, and check for slowdowns during the testing to see if throttling is occuring.

If the cpu can be kept below 80c (plus any leeway for your thermal measurements) then it is usable in that way. dont skimp on your cooling in smaller cases, and maintance it , and it will work for some time. Then the caps will have aged, the board will start acting flakey and you will have to replace it.