Why your built-in refrigerator runs constantly

First: is it actually a problem?

Put an appliance thermometer in the refrigerator and the freezer and read them the next morning. If they hold 40 °F or below and 0 °F, the unit is doing its job. Some newer refrigerators are designed to run long, quiet, low-speed cycles, and any refrigerator runs longer after a big grocery load, a lot of door opening, or on a hot day.

It’s a problem when long run times come with temperatures creeping up, a grille area that feels hot, frost where there shouldn’t be any, or a fan that’s suddenly loud.

Running constantly isn’t the same as cycling on and off

If the compressor keeps switching on and off every few seconds, that’s a different problem called short-cycling. On a Sub-Zero 249FF freezer on a yacht in Everett, the cause was a failed start relay and overload on the compressor, which kept tripping it before it could run a full cycle. This guide is about the opposite: a unit that runs and runs.

The common causes, cheapest first

1. A clogged condenser

The condenser coil behind the grille is where the heat goes. When it’s matted with dust, the compressor runs hotter and longer to do the same job. On a Sub-Zero 736TCI in Bellevue, the refrigerator felt hot along the bottom, and the condenser was buried in dust and lint. Cleaning it restored airflow on the spot. Our Sub-Zero maintenance guide shows how.

2. A door that doesn’t seal

A torn or flattened gasket, a door that doesn’t close fully, or something on a shelf pushing it open lets warm, humid air in all day. The same Bellevue 736TCI also had a door gasket torn along the hinge side. Run a strip of paper around the door: wherever it slides out with no resistance, the seal isn’t doing its job.

3. An evaporator coil iced over

Frost-free refrigerators melt the frost off the evaporator coil on a regular defrost cycle. When the defrost system fails, ice builds up, air can’t pass through the coil, and the unit runs constantly without getting cold. Typical signs are frost on the back wall of the freezer, a loud fan, and an ice maker that stops.

On a Bosch French-door refrigerator in Lake Forest Park, the freezer couldn’t keep ice cream solid, the ice maker had stopped and the fan was loud. The evaporator was packed in ice from top to bottom because the defrost sensor had failed. On a Liebherr CS 2060 in Seattle, the freezer was frosting over for the same kind of reason, and the fix was a new defrost sensor mounted where Liebherr’s instructions specify.

4. Low refrigerant or a failing sealed system

If the refrigerant has leaked out, the unit can run nonstop and never reach temperature. On a Sub-Zero BI-36UID in Seattle, the refrigerator section couldn’t hold temperature and ice was building up inside it. The refrigerator’s sealed system had leaked, and we replaced the compressor, evaporator and filter-drier. That model has separate refrigerant systems for the refrigerator and freezer, so one side can fail while the other keeps working. On a 26-year-old Sub-Zero 700BFI freezer in Woodinville, a leak at the heat exchanger and moisture in the compressor meant a full sealed-system rebuild.

What you can check yourself

1. Read both compartments with a thermometer.
2. Feel the grille area. Warm is normal; hot isn’t.
3. Check the gaskets with a strip of paper, and make sure nothing inside is holding a door open.
4. Look at the back wall of the freezer for a sheet of frost or ice.
5. If you can reach the condenser and the unit is switched off, clean it with a soft brush and a vacuum.

If the temperatures are still climbing after that, the cause is most likely the defrost system or the sealed system, and both need a technician. Sealed-system work requires EPA Section 608 certification.

Is it worth repairing?

On built-in refrigeration, usually yes. Sub-Zero built-ins commonly last 20 to 35 years or more, and replacing a panel-matched unit can mean cabinetry work too. Our repair-or-replace guide goes through how to weigh it. We repair Sub-Zero, Liebherr, True Residential, Thermador and Viking refrigeration, and we’re an Authorized Service Provider for Liebherr and True Residential.