Fan always on, but CPU usage low? Please help

Hi,
my mid-2009 MBP has had the very same problem since just after the warranty expired .

You'll notice that some of the iStat temperature readings will sometimes drop to ridiculously low values (e.g. -297°C) and then disappear (on my machine this happens from time to time). This causes the fans to start spinning at 6200 RPM until the sensors come alive again.

The problem is detected by diagnostics only when the sensors are not working (they show up just fine otherwise).

Unfortunately, the sensors are integrated on the MOBO and fixing them will most likely require replacing it.

http://www.ifixit.com/Wiki/MacBook_Pro_15"_Unibody_Troubleshooting#Section_Thermal_Sensor_Issues

 

Have you tried resetting the PRAM/NVRAM?

 

do a apple hardware test as well. Hold 'D' right after you start up your system or else use the second disc that came with your mac and follow the instructions written on the disc.
Sometimes if a sensor doesn't function the system management controller turns the fan on max to prevent any damages. Apple hardware test will let you know if its a hardware issue or software related.

 

PRAM/NVRAM has nothing to do with fan issues. Resetting it will not help. Only resetting the SMC addresses such issues.

Follow every step of the following instructions precisely. Do not skip any steps.

1. Launch Activity Monitor
2. Change "My Processes" at the top to "All Processes"
3. Click on the "% CPU" column heading once or twice, so the arrow points downward (highest values on top). (If that column isn't visible, right-click on the column headings and check it, NOT "CPU Time")
4. Click on the System Memory tab at the bottom.
5. Take a screen shot of the entire Activity Monitor window, then scroll down to see the rest of the list, take another screen shot
6. Post your screenshots.

 

In my case, resetting the SMC (multiple times) had no effect. Hardware failure.