The Vanner 20‑1050CUL‑DC is a 12 VDC to 120 VAC inverter/charger that supplies up to 1 050 W continuous AC power and includes an integrated 55 A battery charger, transfer relay, and GFCI outlet. Common failure modes are inverter overheating or output voltage sag due to degraded MOSFETs or capacitors, the internal charger failing to engage (often from a faulty charging IC or blown fuse), a stuck transfer relay that prevents shore‑power input, and GFCI tripping caused by ground faults in the AC load or wiring. First‑check diagnostics: verify 12 VDC battery voltage and continuity of the charging circuit; inspect the inverter’s heat sink and fan for proper airflow and check output waveforms with a multimeter or oscilloscope; confirm the transfer relay status and test the GFCI outlet by applying a known load.[oss-gpt-20b]