Sevcon 8055 (P/N 634A85203) is a Gen‑4 Size‑6 motor controller rated at 80 VDC and up to 550 A RMS, designed for high‑power EV or industrial drives with built‑in vehicle control unit and support for AB‑UVW Sin/Cos encoders. Common failure modes on this platform include power‑stage overheating or insulation breakdown (often triggered by overcurrent protection), encoder mismatch or loss of phase signal leading to stall or mis‑driving, firmware corruption causing abnormal error codes, and thermal runaway in the heat sink assembly. First‑check diagnostics: verify input DC bus voltage and ripple against spec; inspect all I/O connectors for loose or corroded contacts and confirm encoder signals are present and within expected amplitude ranges; review the controller’s diagnostic log for overcurrent, overtemperature, or communication errors; finally, perform a visual inspection of the power MOSFET array and heat‑sink mounting for signs of damage or excessive thermal stress.[oss-gpt-20b]