The Siemens A5E03973651 Sigma Control Basic is a compact PLC‑style controller used to manage Kaeser screw compressors and other compressed‑air systems, featuring a built‑in display, programmable parameters, and remote contact control. Common failure points include burnt or tripped relay contacts on the power supply board, damaged input/output modules due to overvoltage spikes, and firmware corruption that can cause the unit to lock up or lose communication with the compressor. For first‑check diagnostics, verify the 24 V DC supply voltage and continuity of the main power rails; inspect all I/O terminals for signs of arcing or corrosion; and review the on‑board error log via the display menu for any fault codes that indicate relay or module faults.[oss-gpt-20b]