New iMac fan keeps running

It took a few days because the problem had stopped, but now it is back. The fan in question is the CPU fa. It normally runs at about 1000 RPM (like the Hard Drive fan and Optical Drive fan. Occasionally, however, it runs at 4000 RPM and makes a lot of noise. Further, it is hard to get it to slow down (sleep mode seems to finally do the trick). The fan gets initiated at 4000 RPM by Flash files, among other things. GPU Diode and GPU heatsink are at 45 degrees C, everything else seems to be at about 35 degrees or less.

Any ideas what I can do about loud fan?

Just looked now and two fans are running over 4000 RPM. CPU and Optical Drive. No idea why Optical Drive fan should be on - have not used drive in many days.

Note: Now Optical Fan went off. Noise still continues from CPU fan. I have done nothing on this iMac since rebooting 7 hours ago except surf internet with Safari

Message was edited by: Perry Gershon

CPU fan runs at 4000. Almost impossible for me to get it to slow down. Reset SMC several times. Any ideas anybody?

Finally ran hardware test

Error 4SNS/1/C0000008:TS2p--124

Will follow-up with Apple

Message was edited by: Perry Gershon

Temperature sensor error. New logic board being installed.

New logic board install - 100% at Apple's expense. Took about a week. Works fine now

If you have a new iMac, there is a way to start it up in hardware diagnostic mode. Older machines require you to have the install DVD. Either way, you want to run Apple Hardware Test and it should tell you if you have a problem.