Zapi AU3503 is a 24‑V/350 A AC motor speed controller commonly used in forklifts and pallet trucks. Common failures include burnt or tripped main contacts on the high‑current relay, degraded electrolytic capacitors in the power stage, overheating of the cooling plate leading to thermal shutdown, and loss of communication with the handheld diagnostic handset due to bad wiring or firmware mismatch. For a first check, verify that the 24 V supply is within spec and that all input/output terminals are clean and securely connected; then use the Zapi handset (or equivalent) to read alarm logs for contactor or capacitor faults, and inspect the cooling plate for excessive heat buildup or corrosion.[oss-gpt-20b]