Loud Fan Won't Turn Off

[Kiko]

I have an HP Desktop 530J and am running Windows XP SP3.

There are three cooling fans, and the loud one only used to go on for a few seconds or minutes when the room got hot or when I was using excessive CPU. Now the loud one goes on as soon as the computer boots up and NEVER goes off. It is really annoying.

I took off the cover and vacuumed all the dust inside and I even took a couple of the fans apart to clean the insides, but the problem persists.

Is there some kind of temp. sensor that could have burned out or is malfunctioning?
Is there a software command I can use to turn the loud fan off?

Edit: Now there is a new symptom. The computer spontaneously turns itself off with no warning. Is that what it does when it thinks it is overheating?

TIA

[patrick88]

Might want to check the cpu fan and heatsink. It sounds like your cpu is over heating.
If it keeps doing this. you will be replacing your cpu.

[Kiko]

It does seem like the rear of the processor is getting quite hot.
I don't know what a heat sink is. Do you think the CPU fan has stopped working?

[patrick88]

Open the tower n to see if the fans are all on.
The heatsink is the round or square thng that has a fan sitting on it. If it's full of dust your cpu will over heat. your mother board is telling you something is over heating. You need to solve this problem soon. If not your spending money. New cpu or new computer.

[LLigetfa]

Chances are it is a PWM fan and the high speed could be one of two things. Either the fan is out of control or there is a call for more cooling. If the pulse wire is open, the fan may run at full speed. I frequently replace those fans but usually because they stop turning.

http://en.wikipedia.org/wiki/Fan_control

You might be able to monitor temp and RPM with SpeedFan. I have used it with several laptops before.

http://www.almico.com/speedfan.php

You might also check for a BIOS update from HP. Also, check out HP's support forum.

http://www8.hp.com/us/en/support-drivers.html

[Kiko]

Okay, I've had to borrow another tower to get back on line.

It was the CPU fan that was running on high speed all the time, and after an hour, the computer would power down.
The problem got much worse, once I took out the CPU fan and heat sink, cleaned them and put them back.
Now it runs on high speed for only a few minutes before the computer powers down.

The case is open when I'm testing this, so I don't know why the board should be overheating. Is there some kind of heat sensor that could be failing? Or is there something in the CPU board that is overheating, even though I don't detect any heat?