Fault Codes/Caterpillar/81 O/UCaterpillar81 O/UUnclassifiedGeneralWhat This Code MeansOver/Under Frequency faultRelated Caterpillar Fault Codes1000-0081348-554-SM50/511357-02532RV1357-040321357-51027/591395-08274507542Browse 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.TRY IT FREE