At the experiments of the CERN Large Hadron Collider (LHC), 31 gas systems are used to deliver a precise gas mixture to the corresponding detectors with high reliability and availability*. The Distribution Modules of these gas systems control and monitor gas flow and pressure across thousands of channels. The Embedded Local Monitor Board (ELMB), an I/O board using the CANopen communication protocol**, is used to read the flow and pressure sensors. After years of operation, the PLC hardware and software of most of the Distribution Modules have become obsolete, raising the need for complex support and impeding improvements. This led to a comprehensive upgrade which was first tested on a retired LHC gas system and subsequently deployed on the production gas system of the MDT detector in the ATLAS experiment. The upgrade allowed the addition of several features, for instance the reporting of detailed sensor error codes and diagnostic information about the ELMBs. System maintainability and upgradability were improved, ensuring uninterrupted operation for the upcoming years of LHC operation.