Why a built-in that is fine at breakfast is warm by five

Heat is a load, not a fault

Refrigeration capacity is not a fixed number stamped on the appliance. It moves with the temperature of the air the condenser is breathing. Take a room from the low seventies to the mid nineties and the same compressor, in the same good order, shifts noticeably less heat per hour than it did, because the refrigerant now has to condense at a higher temperature to lose heat into hotter air. The control has exactly one answer to that: run longer. For most of the summer, longer is enough. The trouble starts when longer runs out, and it runs out first on the unit that was already carrying some small handicap nobody had noticed. Heat does not create the fault. It removes the margin that was hiding it.

• Afternoon sun on a west-facing kitchen, which stays in the room long after it stops falling on the glass
• A range or a pair of wall ovens working across from the refrigerator, rejecting their own heat into the same air
• A condenser coated in dust and dry hillside pollen, which is the usual handicap in this valley
• Cabinetry or a decorative toe kick fitted after the appliance, narrowing the path the air takes in and out
• Boxes, baskets or a platter stored on top of a unit whose condenser breathes through the upper grille
• An installation outside conditioned space: a garage, a butler's pantry with the door shut, an outdoor kitchen under a solid roof

What dust actually does to a coil

A condenser is a run of tubing with thin fins pressed onto it, and all of its work happens where those fins meet moving air. A film of dust, pet hair and pollen does two things at once. It insulates the fin, so heat crosses it more slowly, and it narrows the gaps the air has to pass through, so less air arrives to carry that heat away. Neither shows up as a warning: the appliance holds its number and pays for it in run hours. The only honest way to see it is to look at the coil, or to compare the temperature of the air going into the grille against the air coming out. A clean coil is the cheapest thing on this page. What eventually happens to a compressor that spent three summers covering the difference is the most expensive.

• Kill the power first, at the breaker or the appliance switch. A condenser fan can start while your hand is in the grille.
• Use a soft brush and a vacuum, working along the fins rather than across them, because a bent fin closes the gap you are trying to open
• No pressure washer, no hose and no wet spray into the machine compartment; there are live connections and a fan motor in there
• Find where your unit actually breathes: most modern built-in columns take air through the grille above the doors, and undercounter and drawer units through a grille at floor level
• Check the clearance while the grille is off, above the unit and behind the toe kick, and put nothing back that was not there originally

The readings that separate a hot room from a broken machine

All of this is settled in about twenty minutes with a meter, a thermometer and a clock, and none of it involves opening the sealed system. The cabinet answers the first question by itself. On the built-in and column units that use dual refrigeration there are two separate sealed systems with their own compressors and evaporators, and no air passes between the fresh food and freezer compartments at all, so a freezer still sitting hard while the refrigerator climbs has already cleared half of the machine. What is left is airflow, defrost, a fan, a sensor, or a condenser that cannot breathe. Each reading rules something out rather than confirming a hunch, and a fault that only appears after lunch has to be measured after lunch.

• Air temperature into the grille and out of it while the unit runs, read together with how much air is actually moving: a coil insulated with dust warms the air crossing it very little, while a blocked grille or a slow fan sends a thin stream of very hot air out instead
• Current draw on the condenser fan, with the blade checked for a wobble or a rub. A fan turning slowly is worse than one that has stopped, because nothing about it looks wrong.
• Current draw on the evaporator fan for whichever compartment is warm, plus a look for ice touching the blade
• Defrost heater continuity measured cold, and thermistor resistance compared against the curve published for that model at the temperature it is actually sitting at
• Cabinet temperature from a calibrated probe at the coil and from a thermometer in a glass of water on the shelf, rather than from the display
• Run time watched across a full cycle, with the compressor housing checked for how hot it is running

When it really is the sealed system

Sometimes it is, and the tell is that the pattern stops tracking the room. A capacity problem caused by heat gets better overnight, better in October, and worse in a heat run. A sealed system in trouble does not care what time it is: the unit runs constantly in cool weather too, the compressor is hot to the touch, and in the classic case it tries to start, trips its overload and clicks again a few minutes later. This is where the work stops being a homeowner job. Putting gauges on a sealed system and handling the refrigerant in it require EPA Section 608 certification, and a system genuinely short of refrigerant is short because it has a leak, which means the leak has to be found and repaired rather than the system topped off. A recharge with no leak repair buys a season at most, and the second visit costs what the first one should have.

• Ice or frost forming on part of the evaporator only, with the rest of the coil bare, which points at a charge or a restriction rather than at defrost
• A compressor that starts, runs for a moment and stops repeatedly on its overload
• Cooling that never fully recovers even in cool weather, on a unit whose coil and fans have already been cleared and read
• Oil residue or a dark, dusty film around a joint or a service port, which is often where a slow leak has been announcing itself for months

What is cheap, what is expensive, and what to do before the visit

The reassuring part of this class of equipment is that the common answers are all small parts. A condenser cleaning fixes more inland cooling complaints than every electronic component put together. Behind it come the little motors and sensors: a condenser fan motor, an evaporator fan, a defrost heater, a thermistor, a door gasket that has lost its magnet. Every one of those fits in a hand and every one is a single visit. Above them sit the control boards, which are blamed far more often than they fail. At the top is the sealed system, and that is the expensive one. The whole argument for cleaning and measuring first is that a heat-choked condenser imitates a dying compressor almost perfectly, and the two repairs are not remotely in the same league.

• Write down the time of day the problem appears and the time it clears. That one sentence changes the order the tests are run in.
• Never leave a door open to help it recover: you are loading the coil with moisture it has to freeze out later
• Put a thermometer in a glass of water on the middle shelf and read it morning and evening. A glass of water tells the truth about the cabinet; a display tells you about the display.
• If a code is showing, write it down before it clears itself
• Book the appointment for the part of the day the fault actually happens, if it has a time of day at all