Elgo P9511/P9521 is a compact single‑axis position controller designed for use with 2‑ or 3‑speed bi‑directional drives or variable‑speed drives offering 1, 2, or 4‑quadrant control. Common failure points in this class of controllers include power supply degradation (especially the isolated DC bus), burned input/output relays, and firmware corruption due to improper updates or electrical noise. Typical symptoms are loss of position accuracy, intermittent start/stop behavior, or error codes indicating I/O faults. First‑check diagnostics: verify 24 VDC and 48 VDC supply rails with a multimeter; inspect all relay contacts for corrosion or burn marks; review the controller’s diagnostic log for any recent fault entries before proceeding to deeper firmware or PCB inspection.[oss-gpt-20b]