Over the years as a SLC 500 PLC programmer, the first thing I look for when a PLC has failed is the front face of the CPU. The LED gives me an insight of what is going on system wise, even without using the RSLogix500 software. The LED That I am referencing to are as shown below.
The table below has been my “go-to cheat sheet” to decipher the CPU LED lit and flashing sequence. I hope the table would serve you the same way as it did for me. The table below is from Allen-Bradley’s “Ethernet SLC 500 Processors – Quick Start for Experienced Users” (page 3). This document can also be found in the Reference link below.