We have had the same problem with some of our 520/620's. We had dell send us a new fan and they fixed the problem.

My company have this problem on a few of our 620s.

I was able to solve it on 1 of them so far. what i did was:

1. Replace the motherboard battery(it was empty)

2. Update the bios

3. Get rid of the dust inside the cabinet(making sure to get rid of the dust on the fans)

4. Update the graphics card.

Not sure what fixed it but so far i have no complaints on it after i did those things.

I was having the same problem after I installed a Zotac 8400GS graphics card. I went crazy looking for the problem - changed the fan, heat sink, CPU, power supply and battery without results. After reading your reply about the graphics card I uninstalled the card - problem was solved. The Zotac card runs way to hot for this system.

I've been using the internal board graphics - seems okay for my use. I still get noticeable  increased fan speed when the CPU is working hard ( Pentium D 3.2 GHz) but not even close to when I had the graphics card installed. I guess the GX620DT just has this problem.

A fanless Radeon 5450 that only uses 20w of power may be the cure.

This is a GX620 DESKTOP. I thought the copper heat pipe performance heat sink was available for mini towers only.

I do have a copper piped heat sink ( which I believe is standard) in my GX620 desktop.

A Pentium D should have the Copper heat pipe performance heatsink.

The 620 FAQ Covers this.

DELL OptiPlex GX620 - FAQ - Frequently Asked (and answered) Questions

I have been using a GX620 for a few months and noticed the variations in fan speed. It gets noisy at times and I use my earphones to reduce this issue. I regularly remove the dust from the inside of the case, including the fans. A couple of days ago I turned to computer on to complete fan silence. The computer functioned normally, except no fan activity. I removed the cover and the case fan was not turning. I also noticed the power supply fan was not turning. It could be the power supply fan stopped function at an earlier time and I just did not notice it. I removed the power supply and removed the cover and removed the fan, I tested the fan with an external 12VDC power source and the fan worked normally, I installed the fan back in the power supply, put the cover back on, installed it, and turned the computer on and the fan still did not turn. The power supply provides +/- 12VDC to the motherboard, and the computer works normally. It is a mystery to me why the power supply is failing to provide 12VDC to its own cooling fan.     I have not yet had time to look in to the case fan being inoperative. All I know is that it is a 4 wire 12VDC fan with sensor and control wires that have to be connected to a temperature controller, I am not sure if the fan is bad or the temp controller. I am not sure where the temp controller is located. If anyone could point me in the right direction I would sure appreciate it. Thanks,

I don't know which size GX620 your using, but I had the same problem with my GX620 Mini tower ( the power supply fan stopped working). I replaced the power supply to fix it. I have all sizes of the GX620's and all of them increase fan speed at times especially when you first turn them on. Unfortunately, I believe, the heat senor control is part of the motherboard. I also, had to replace the motherboard in a GX620 DT due to the CPU fan control not working properly.

Wish I had better news - hope this helps

I have a GX620 Small Form Factor. Going the way you did and replacing the motherboard and the power supply seems to be the expensive way to go. I already tested the power supply fan and and it tested good, so the problem with the power supply fan is internal to the power supply. I already have a 12VDC, 3.25 amp power module I can use to externally run 12VDC power to the power supply fan. This is more cost effective and probably much more reliable and cost effective than me purchasing a replacement power supply and have the same thing happen again a few months down the road. When the replacement case/cpu fan arrives, and if  the problem turns out to be a sensor/control motherboard problem. I can route 12VDC to the case/cpu fan from this same external power module.  This would also be more cost effective than purchasing a replacement motherboard and have the same problem happen again in a few months. I came across a youtube video showing how the air being blown across the cpu heat sink becoming extremely hot and then being forced across the top of thr motherboard causing capacitors to expand and fail. The suggestion was to turn the fan around and have the case/cpu fan blow the hot air out the front verses in across the top of the motherboard. This just makes common sense since the cpu heat sink does not care which way the air comes across it. I am also considering installing another 80 mm fan on the rear of the computer case blowing cool air inward to help the original case/cpu fan after I reverse it. I will mount this additional fan externally in the area of where the hard drive is located. I will let the air from this fan find its way around the components.  As many of us know, heat is the enemy of computers, and the less the better.  Thanks for your reply.