Fault Codes/Rehlko/516663 31Rehlko516663 31UnclassifiedGeneralWhat This Code Means516663 31Possible Causes•NOx sensor 4 (downstream SCR) removed from exhaust lineRelated Rehlko Fault Codes516666 1516667 31516663 0A-227600516600 4ACCY PWROVER WARNING516600 3AD516284 17AHET516281 15ALOPBrowse all Rehlko fault codes→ASK RUDY ABOUT THIS FAULTGet a diagnosis walkthrough for 516663 31 on your specific Rehlko unit — cited to the OEM manual, not a forum guess.Your question about 516663 31GET THE ANSWERFree account required — we'll bring your question with you.