Fault Codes/Rehlko/516666 1Rehlko516666 1UnclassifiedGeneralWhat This Code Means516666 1Possible Causes•DEF level too low for coolingRelated Rehlko Fault Codes516667 31A-227600516663 31ACCY PWROVER WARNING516663 0AD516600 4AHET516600 3ALOP516284 17AUXBrowse all Rehlko fault codes→ASK RUDY ABOUT THIS FAULTGet a diagnosis walkthrough for 516666 1 on your specific Rehlko unit — cited to the OEM manual, not a forum guess.TRY IT FREE