In most cases, this means the internal protection has been exceeded (tester has seen greater than 15A), which normally occurs when accidentally testing more than one string at the same time. Make sure there aren’t two strings in parallel connected to a “Y” splitter before the cabling enters the combiner box. Also, check that the system had been fully isolated, and that only one fuse has been closed at any one time during string testing. Depending on the extent of the damage, sometimes the FET error will only be displayed when connected to a string and pressing the AUTO test button. An audible bleep will also be heard. If the FET shows when powering the tester up and not connected to a string, damage is more extensive. Tester needs to be returned for repair in both situations.