The short answer: most boat fridges that stop cooling are being starved of voltage rather than broken. The compressor controller shuts down when voltage at its terminals falls below a threshold, and on a boat that happens through low batteries, undersized cable or a corroded connection long before the fridge itself is at fault.
Start with the supply, then ventilation, then the unit. That order finds the answer quickest and avoids replacing a compressor that only needed a better cable.
Measure voltage at the compressor controller while the compressor is trying to run, not at the battery with everything idle. If the battery reads healthy but the controller sees much less under load, the loss is in the cable run or its connections.
Fridge controllers are deliberately protective. When the voltage dips at start-up they cut out, wait, and try again. To the owner, that looks like a fridge clicking on and off and never getting cold.
Many fridges were wired with cable sized for the running current and nothing more, sometimes over a long route from a distant panel. The start-up surge then drops enough voltage to trip the controller. Replacing the supply with correctly sized cable is one of the most common fixes we carry out on refrigeration faults.
Fridges often live in damp lockers near the galley. Green crimps, loose screw terminals or a corroded fuse holder all add resistance that behaves exactly like a thin cable. They can also run warm, which is a warning in its own right.
A fridge is usually the first thing to complain when a bank is low or ageing. If the fridge fails mostly overnight or after a day at anchor, the real issue may be battery capacity or charging. A battery monitor makes this obvious in a way the fridge never will.
Compressors shed heat through a condenser that needs airflow. Boxed into a tight locker with no air path, the system runs longer, draws more energy and cools less, especially in summer. Clear space around the condenser and check any cooling fan is actually turning.
Once supply, batteries and ventilation are ruled out, the controller, thermostat or refrigeration circuit may be the issue. Some controllers report fault codes through a flashing LED, which is worth noting before anyone visits.
Safety note: fridge supplies are low voltage, but the battery bank feeding them can deliver a very large fault current. Any rewiring back to the batteries should be fused correctly at the source; if you are unsure, that part is best left to a qualified marine electrician.
Charge the batteries fully and confirm they hold voltage. Measure at the compressor controller while it starts. If voltage collapses there, inspect the fuse holder, switch and every joint on the supply, then consider the cable size. If the supply is sound, clear space around the condenser, clean it and check the fan turns. Only after those steps does it make sense to suspect the controller or refrigeration circuit.
Writing down the voltages you measure, and when, is surprisingly useful. It turns a vague complaint into evidence, and it means that if you do call someone in, much of the diagnosis is already done.
Once the fault is cured, a few habits reduce the fridge's share of your battery budget. Pre-chill food and drink on shore power before heading out. Keep the lid or door closed as much as possible and avoid packing warm items in all at once. Better insulation around an older cool box can cut running time considerably.
A battery monitor shows the difference these changes make in amp hours rather than guesswork. On boats that spend long periods at anchor, the fridge is often the load that decides whether the bank and charging are adequate, so it deserves attention when planning any battery upgrade.
That pattern usually means the controller is cutting out on low voltage during start-up. Check the voltage at the compressor while it tries to run.
That points strongly at the batteries or DC supply rather than the fridge. On shore power the charger props the voltage up and hides the weakness.
Rarely as a first step. A new fridge on the same inadequate supply will behave the same way. Fix the supply, then judge the unit.
We trace refrigeration faults from the battery to the compressor and put the supply right. Free assessment, then a written quote.