What does ‘SCR NOx conversion efficiency low’ mean on Detroit Diesel DD13, DD15, DD16? Fault codes associated with Selective Catalyst Reduction (SCR) NOx efficiency low are the most dreaded codes you will find on the Detroit diesel application and can drive your costs high. Long story short, these fault codes mean that the efficiency in the aftertreatment system, aka the 1-Box, is not operating within the required threshold (70%). This measurement happens between your inlet and outlet NOx