Warning W2518 - Mainboard temperature is too high (i.e. over 50┬║C). This should appear only if the mainboard has a temperature-monitoring sensor installed.
Fix: Power down the system immediately. Open the case and check that all the fans are working and check whether components are unusually hot inside. Make sure the components have good all-round ventilation. Use more fans if required.
For more information, please see the (FAQ).