Cpu fan stuck at max speed

Question

Out of nowhere, my computer (early 2006 20" Intel iMac) started making huge fan noises. The iStat pro widget shows that the CPU fan is blowing at 3600 rpm (apparently the max), and the other two fans are normal. CPU usage is minimal, and CPU temperature is low (30-40 degrees C), so there's no good reason for the fan to go to the max.
I've tried (several times) to reset the SMC, but it doesn't help. A couple times over the past five days, it's gone back to normal, but then it soon goes back to max.
Anybody experienced anything like this? Any suggestions?
Thanks in advance,
Neil

Answer 1

I have the same problem,
I know a lot about computers (for not a professional) and after doing anything i can without opening the case,(SMC reset, Vram reset, safeboot and starting on verbose,Apple hardware test)
In my specific problem the facts are
4SNS/1/40000000:'TmOP' from apple hardware test
Cpu Fan: 3604rpm (really high) this is the logic board fan
the other 2 fans are normal
Cpu tmp: 28 C (low)
GPU diode 127 C (ridiculously high - no tmp variation) from Istat pro
Memory controller 127 C (ridiculously high - no tmp variation)
all the other temperatures, as the cpu ,are a little bit lower than normal which is normal because of the high speed fan
that means logically that the system is reading a 127 Celsius tmp in these places
and the fan is responding right
So the tmp sensor is sending a false tmp
there are 3 sensors this one is on the logicboard and you cannot change it
i went to a certified apple service provider and we open the case together,
as i wanted to be there.
I hoped with some dust cleaning or something position related we could do something but no, after checking every possible solution,
needs a logiboard replacement that he (like me) did not recommended (very expensive)
Other than the fan noise there is(until now) no other symptom, in OSX or XpPro
so I'm not going to pay about 700 euro (for everything) for this machine
I'm positive (i checked it myself) that there is nothing hot inside, just the sensor circuit is stuck at these tmp
If the the fan dies sooner its much cheaper and i can even replace it myself
I 'm very sad and disappointed with this solution but what can i do?
It is just 2 months after the warranty! and i know that this is a logicboard defect
the ironic thing is that before that happened i was thinking o upgrading the Cpu 2.0CD to a 2.33C2D (yes you can, if you really want) but now i 'm going to leave it like this and sometime buy a new thing (not an imac again!) a mac pro if i am richer or a decent PC
I really like macs and everything, but the compact feature when the time comes may have a compact problem!
the other funny thing is that they offer me a job in the Service place but i am an artist
Good luck and i hope someone finds a solution or something else