Optiplex 745, 305W PSU, Core 2. 2.1 gz can I upgrade to 755 Motherboard

The amber LED on the motherboard is a good thing!

That LED should be on 24/7, as long as the PC is connected to a working outlet, and not turned off at some power strip.

Are you talking about the CPU fan? The CPU fan may spin, but Dell OEM fans have a sensor in them that has to be detected by BIOS. If the sensor fails or if you use a fan without that sensor, BIOS won't be able to control fan speed. Reboot and press F12. If you have the option listed on the F12 menu, go to Utilities partition and run the fan diagnostic tests.

It's also possible the thermal paste between heat sink and CPU has dried out. In this case, you have to remove the heat sink, clean both surfaces and apply a thin coat of fresh paste, eg Arctic Silver, and then reattach the heat sink.

And it's also possible your video card may be failing...

As for swapping boards, Dell typically uses a custom wiring connector between the front panel and motherboard for each different model. So even if the other board fits into this case, you may have problems connecting the board to the front panel, unless you're prepared to map the wiring.

Great on the amber light, I get no trouble codes off the front or beeps, I just get the CPU fan failure, replaced the fan, still nothing, tested both fans as per another forum, with a 9v battery, both spin up. I have had problems wiht this motherboard with USB sometimes. I did find a problem with the video card, the heat sink popped a couple of pins, I ordered a new one, & resat the heat sink, it's got zip ties on it, working till the new one comes in. GPU temp is back down, computer is not crashing now when playing video. Ordered a board that is suppose to fit a 745, 745c & 755. They all take the same power supply & chip, was just wanting the 755  option of a quad core & total 8 gigs of ram. I will try to re-seat the CPU, I have thermal paste, I also have another Pentium Chip that will run in this, so I eliminate the CPU, just have things on Ebay & Amazon, bad time for the only computer to go down. Thanks for the tips, I will follow the bread crumbs & see what I can find.

Before you update the CPU on either the old board or on the replacement board, be sure to update BIOS to the latest version for that board that's available on Dell's support page. Obviously, if you're installing a 755 board you'll want to look on the support page for the 755, not the 745 for BIOS updates.

If you replace the CPU chip without the latest version of BIOS, it might not work because the older version might not support the newer chip. So do the update with a chip you know will work.

If possible remove the heat sink without removing the CPU so you don't risk bending its pins. You should still clean the top of CPU and bottom of heat sink to remove old paste and then apply fresh paste to the top of the CPU and reattach the heat sink.

Good luck with the upgrades!

Thanks Rohe, went right in, looks identical, same slots, it says it's a 745C model, I notice a little speed up, but it's the same ram & chip, just replaced the video card that was going out, fan in it was loud & running a little hot. I can't find the info on this board, people I bought it from, not much help. Thanks for tips, got the cpu thermal paste, it was the board, cpu fan fire right up, I don't see any blown or swollen caps on the old board, I would try to tinker with it if they were oviously bulging.