From the Apple discussion boards:
We are aware of an erroneous fan error that Apple Hardware Test (AHT) may report on Power Mac G5s. If you run AHT and receive either of the following errors:
2Fan/4/x : Fan For BackSide: This error code usually indicates the Drive Bay has failed. The Drive Bay fan is located between the hard drive(s) and SuperDrive.
2Fan/4/x: Fan For Slot: This error code usually indicates the PCI Slot fan has failed. The PCI Slot fan is located in front of the PCI slots near the front of the computer and just under the SuperDrive.
Do the following:
1. Remove the outer door and verify the Drive Bay fan and the PCI slot fan are spinning (If the fan is spinning, there is nothing wrong) 2. If either fan is not spinning, take the computer to an Apple Authorized Service Provider for service
Note: AHT v. 2.1 has been updated to v. 2.1.1 and contains a revised fan test. The new version of AHT will be available soon.
Mark Apple Support