iMac - iMac G5 automatically goes to sleep mode

My Apple iMac G5 (OSX 10.4) starts up as normal, runs for about 5 minutes and then automatically goes to sleep. It will wake up if prompted, but every time it goes back to sleep in seconds. If left in sleep mode for a few minutes it will wake up again for 1 – 5 minutes before once again automatically returning to sleep.

It is definitely not due to hot corners. I changed the power saver settings several times and also reset the SMU but with no luck. I’ve also ran an apple hardware test which got the fallowing error code: “2FAN/4/8: CPU Fan”

Anything else I can try besides taking it in to an Apple service provider?

Thank you!