Fault Codes/Caterpillar/81 O/UCaterpillar81 O/UUnclassifiedGeneralWhat This Code MeansOver/Under Frequency faultRelated Caterpillar Fault Codes1000-0081000; 741832RV1051; 1054-037321063-04027/591064-5711078-0111078-5351152-5351348-554-SMBrowse all Caterpillar fault codes→ASK RUDY ABOUT THIS FAULTGet a diagnosis walkthrough for 81 O/U on your specific Caterpillar unit — cited to the OEM manual, not a forum guess.Your question about 81 O/UGET THE ANSWERFree account required — we'll bring your question with you.