At firealarm.com, we’ve extensively tested the Notifier FMM-101 as a replacement for the System Sensor M501M, and we’re impressed with its performance. The FMM-101 offers enhanced features such as improved sensitivity and faster response times compared to the M501M. Additionally, its compatibility with modern fire alarm systems makes it a seamless upgrade choice. Overall, we highly recommend the Notifier FMM-101 for its reliability and advanced functionality.