12/24V marine refrigeration diagnostics

Marine DC fridges (Isotherm, Vitrifrigo, Nova Kool, Waeco) use small hermetic compressors (Danfoss/Secop BD35 or BD50) driving R134a or R600a refrigerant. Typical current draw: BD35 ~2.5A at 12V running, BD50 ~4.5A. Fault codes come via a blinking LED on the compressor control module, count the flashes: 1 = electronics failure, 2 = fan overload, 3 = motor start failure, 4 = compressor overload, 5 = thermal overload. Most common real-world failures: (1) Insufficient airflow to the condenser, a dust-blocked or heat-soaked condenser will cause thermal shutdown. Clean the condenser fins with compressed air. (2) Low voltage cutoff, Danfoss modules cut off around 10.4V (or 21V for 24V systems). If your battery sags below that under load, the fridge stops. Check with a voltmeter at the compressor terminals under load. (3) Failed condenser fan, replace the fan, not the compressor. (4) Refrigerant loss, hermetic systems shouldn't leak, but a fitting can fail. Refrigerant work requires an EPA card. If you're chasing a "warm fridge" complaint, always start with voltage at the compressor and airflow at the condenser before assuming refrigerant.