Why is my wine cooler not holding temperature?

The reasoning behind that

In a downtown condo the wine column is frequently out in the living space against floor-to-ceiling glass, where a west-facing exposure puts direct sun on the door for the last hours of the afternoon, and the cabinet loses ground every day and takes it back at night while the display holds its set point the whole time. Where one zone holds and the other climbs, the fault belongs to the climbing zone alone — its evaporator, its fan, its damper — and the zone still doing its job is your proof that the compressor and the sealed system are fine.

How the diagnosis goes

The checks are airflow first, then the door, then the zone. Where the grille discharges matters enormously on a cabinet installed in an island or a bar run, because a condenser exhausting into a closed cavity is fighting its own hot air. After that comes the door: a seal that has gone hard, or a pane heater that has failed and left condensation fogging the glass. Only then do we look at the drifting zone's own fan, damper and evaporator.

Drift matters more than the absolute number on a cellar that is left unattended. Several degrees of swing every day for a whole summer, in a unit closed up while the owner is away, works the cork far harder than a cabinet sitting steady a couple of degrees off. If the column is going to be alone for a long stretch, get the airflow and the temperature verified before you leave rather than after you get back.

If that matches what yours is doing, say so when you call — it decides which parts come on the van.