Fault Codes/Caterpillar Inc./81 O/UCaterpillar Inc.81 O/UUnclassifiedGeneralWhat This Code MeansGenerator over/under frequencyRelated Caterpillar Inc. Fault Codes1052-0401063-04050/511064-571461078-01132RV1078-53532/32RV1152-535321371-040Browse all Caterpillar Inc. fault codes→ASK RUDY ABOUT THIS FAULTGet a diagnosis walkthrough for 81 O/U on your specific Caterpillar Inc. 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.