The Lincoln Commander 400 Control Circuit Board is the main logic and power distribution module for the Commander 400 welding system, integrating power‑supply regulation, I/O relays, and safety interlocks. Common failure modes include blown or tripped fuses on the 240 VAC auxiliary circuit, damaged MOSFETs or relay contacts due to overcurrent, and solder joint cracking from vibration or thermal cycling; these often manifest as loss of power output or erratic welding current. First‑check diagnostics should verify that the 45 A double‑pole breaker is not tripped, inspect all fuses and PCB traces for visible damage, and confirm proper voltage at the board’s input terminals before proceeding to component replacement or reflow repair.[oss-gpt-20b]