The Jandy RS3500 (AquaLink RS) is an integrated pool‑control board featuring an LCD touch interface that manages pumps, heaters, chemical dosing, and auxiliary accessories while providing diagnostic readouts. Common failures include power‑supply dropout or voltage spikes damaging the main microcontroller, LCD backlight or driver failure causing display loss, and relay/auxiliary contact wear leading to intermittent equipment control; firmware corruption can also trigger “system error” codes. First‑check diagnostics: verify 24 V DC supply continuity and output levels at the board’s power terminals, inspect the LCD panel for loose connections or burnt traces, and review the on‑screen diagnostic log for recent fault codes before proceeding with component replacement or reflash.[oss-gpt-20b]