The Core Operating Limit Supervisory System (COLSS) digital monitoring system relies on signals from fixed in-core detectors to determine the peak linear heat rate of the fuel and the departure from nucleate boiling ratio thermal margin. Experience with this type of detector indicates that some failures will occur during operation. Since detector replacement prior to refueling requires costly shutdowns, it is important that the COLSS algorithms be relatively insensitive to a reasonable number of failures. This paper describes the cause-and-effect relationship between detector failures and those COLSS parameters determined by the detector signals, the statistical methods used to determine the additional penalties incurred by detector failures (1% for up to a 25% failure rate), and the proposed limiting conditions for operation for the in-core detector system.