The shapes a failing cooler or freezer leaves in its temperature log
By Vimal Bhaya, Founder and Lead Systems Architect · Last reviewed 21 October 2026
Most refrigeration faults change the shape of a box's temperature readings before the food is at risk. Each shape points to a short list of causes.
A straight climb means the cooling has stopped. A ripple that stops means it cannot catch up. A coldest point that creeps up each day means ice is building on the coil. TemperatureWise watches for these shapes in every box, day and night, and emails, texts, then calls you when one appears.
Normal is a steady ripple, and a freezer adds a few planned bumps a day
A healthy cooler or freezer never holds one exact temperature. Its readings move in a pattern that repeats, and every box has its own.
- The ripple. The cooling runs until the box reaches its setting, then rests while the air slowly warms, then runs again. Each run and rest is one cooling cycle, and each cycle draws one tooth of the ripple.
- The defrost bumps. A freezer's coil runs below freezing and gathers frost. A few times a day the freezer stops cooling and melts that frost off. The air warms for a short while, and the frozen food barely moves. Each defrost draws a bump that comes back down.
- The door spikes. In busy hours, a long door opening makes a spike, and a healthy box pulls it back down.
The size of the ripple, how often it comes and when the bumps fall are different in every box. A two-door reach-in holds little air, so it swings about 5°F (3°C), about twice an hour. A walk-in cooler on a quiet night ripples by about 1°F (0.5°C), about four times an hour. A normal day for each is on how a reach-in cooler works, how a walk-in cooler works and how a walk-in freezer works.
Every fault shape below is a change from that box's own normal. That is why a box has to be learned before it can be watched.
Eleven shapes cover most refrigeration faults, and each points to a short list of causes
A refrigeration fault changes the pattern before it ruins the food. The cooling stops, or runs without a break, or loses a little ground each day. Each of those draws its own shape in the readings.
TemperatureWise watches for these shapes in every box. When one appears, it emails you the shape it matched and the likely culprit. Here it is for two of the shapes below: a straight climb in a parked reefer, and a coldest point that creeps up in a walk-in freezer.
Here are six of the shapes, from the real readings of four boxes: a parked reefer, a restaurant reach-in, a coffee shop walk-in cooler and a distribution center walk-in freezer.
The table lists all eleven, grouped by how fast each one shows. Open "Where it is covered" under any shape for every symptom it shows up under on our troubleshooting pages, with its reasons and the checks you can make.
| What the readings show | What it usually is |
|---|---|
| Shows within hours | |
| A straight climb. The ripple stops and the line climbs toward room temperature, never turning back. Chart 1. | The cooling has stopped: no power, a trip, a compressor that will not start, or a reefer that will not restart.
Where it is covered: 5 linksWalk-in cooler: the unit trips, hums, or will not startWalk-in freezer: the condensing unit trips on hot days Reefer: the unit shuts down, or will not restart; a parked trailer on shore power stops cooling Part page: the compressor and its starting parts |
| Flat, with no ripple. The line jumps, then holds flat well above the setting. Chart 2. | A door left ajar or propped open. The cooling runs nonstop and only holds even.
Where it is covered: 8 linksWalk-in cooler: condensation, wet walls, a door that will not sealWalk-in freezer: frost round the door, or a door frozen shut Reach-in cooler: the door will not close or seal; it runs all the time Reach-in freezer: the door freezes shut, or frost builds round it; it runs all the time Prep table: the base cabinet is warm Part page: the door, gasket, closer and heaters |
| The ripple stops above normal. The line goes almost flat a few degrees above normal and stays there. Chart 3. | Cooling that cannot catch up: an iced coil, a stopped evaporator fan, or a door not fully shut.
Where it is covered: 9 linksWalk-in cooler: the box is warm and will not hold temperature; ice on the coil; runs all the time, or clicks on and offWalk-in freezer: not cold enough, stuck at 10°F to 20°F (-12°C to -7°C); runs all the time, or clicks on and off Reach-in cooler: the cooler is warm and will not hold temperature Reach-in freezer: the freezer is warm and will not hold temperature Prep table: pans and base warm together Part page: the evaporator fans |
| A line that falls below the setting. The ripple runs at the wrong level, or stops while the line keeps falling. | A setting bumped, a sensor moved, or a control, valve or contactor stuck on.
Where it is covered: 6 linksWalk-in cooler: freezing the foodWalk-in freezer: colder than its setting Reach-in cooler: it freezes the food Prep table: frozen food at the bottom of the pans or in the base; lettuce frozen where it touches the walls Part page: the thermostat, controller and pressure switches |
| Shows over days | |
| Defrost bumps change or go missing. A freezer's planned bumps run longer, get taller, come at odd hours or stop. Chart 4. | A failing defrost timer, termination switch, fan delay or heater, or a clock reset by a power cut.
Where it is covered: 6 linksWalk-in freezer: fans off after defrost, or defrost at odd timesReach-in freezer: warm-ups that run long, come at odd hours or stop Display case: it reads warm around its defrosts Reefer: a defrost alarm, ice at the drains, or defrosts too often Part pages: the defrost timer; the termination and fan delay switch |
| The coldest point creeps up. The low point between defrosts or cycles ends a little warmer each day. Chart 5. | Ice building on the coil: a defrost that no longer clears it, a frozen drain, or wet air.
Where it is covered: 12 linksWalk-in cooler: ice on the coilWalk-in freezer: ice building up on the coil; not cold enough, stuck at 10°F to 20°F (-12°C to -7°C); ice on the floor, icicles under the unit Reach-in cooler: ice behind the fan guard that does not clear Reach-in freezer: ice behind the fan guard, or frost on the walls and food; ice or water in the bottom, or on the floor Prep table: weak air from the rail's vents, or ice on the coil Display case: weak air from the honeycomb, or frost on the product Reefer: the load will not hold temperature Part pages: the defrost heaters; the drain line and drain heater |
| Warm on the coldest nights. The box slips on cold nights and early mornings, and is fine by the afternoon. | Failed winter controls on a condensing unit outdoors or in an unheated room.
Where it is covered: 2 linksWalk-in freezer: warm on cold nights, fine in the afternoonPart page: winter controls |
| Warmer every hot afternoon or every rush. The box holds at night but slips in the heat of the day or in service. | A dirty condenser, a hot room, a leaking door or gasket, open lids, or too much warm product.
Where it is covered: 13 linksWalk-in cooler: the box is warm and will not hold temperature; condensation, wet walls, a door that will not sealWalk-in freezer: frost round the door, or a door frozen shut; the condensing unit trips on hot days Reach-in cooler: the cooler is warm and will not hold temperature; an undercounter runs warm on the cookline; the door will not close or seal Prep table: warm food at the top of the pans, over a cold base; warm food in the rail, over a cold base; it runs all the time, warmest in the afternoon Display case: the case will not hold temperature Reefer: the box warms at each stop, or one end of the load is warm Part page: the condenser coil and fan |
| Shows over weeks | |
| The whole day runs warmer. The same cycles at the same hours, but the whole line sits higher than last month. Chart 6. | Lost capacity: a slow refrigerant leak, a worn compressor, a clogging condenser, or leaky walls.
Where it is covered: 9 linksWalk-in cooler: the box is warm and will not hold temperature; runs all the time, or clicks on and offWalk-in freezer: not cold enough, stuck at 10°F to 20°F (-12°C to -7°C); runs all the time, or clicks on and off Reach-in cooler: the cooler is warm and will not hold temperature Display case: the case will not hold temperature Reefer: the load will not hold temperature Part pages: refrigerant leaks and charge; wall panels, floor and pipe holes |
| Needs a second sensor, or shows as silence | |
| Two sensors in one box drift apart. Where a box has two sensors, one end or zone warms while the other holds. | Air not getting round: a stopped fan, a blocked air path, a broken air curtain, or a bad load.
Where it is covered: 7 linksWalk-in cooler: the box is warm and will not hold temperaturePrep table: warm food at the top of the pans, over a cold base; warm food in the rail, over a cold base Display case: the top rows or the front are warm Reefer: the load will not hold temperature; the box warms at each stop, or one end of the load is warm Placement: where to put a sensor in a walk-in |
| No readings at all. The line stops, so nobody can see the box. | A power cut, the internet or gateway down, or a flat sensor battery.
Where it is covered: 2 linksOutages: what happens when the power goes out; what happens when the internet goes out |
Our seven troubleshooting pages cover 51 symptoms and 201 named reasons. 44 of those symptoms leave one of these shapes in the readings. The other 7 you see before any sensor does, as the last section explains.
TemperatureWise watches every box for these shapes, day and night, and calls you when one appears
Picture a temperature analyst who has studied each of your boxes for weeks. They know its ripple, its defrosts and its busy hours. They watch every reading as it arrives, through nights, weekends and holidays, and they know whom to call. That is the job TemperatureWise does.
- It learns each box. TemperatureWise uses machine learning on the full temperature history of each box. In the first week it learns that box's own ripple, defrosts, busy hours and recovery.
- It watches every reading. Each new reading is checked against that box's own normal, around the clock.
- It emails you first. When a shape starts to form, an email says what has changed and the top three likely parts at fault. It can go to as many people as you like.
- It texts if things keep getting worse. A text or WhatsApp message goes to up to three people per sensor.
- It calls until someone answers. The last step is a phone call to the same people, in order, until someone picks up.
A sensor or gateway that goes quiet raises an alert the same way, because silence is not a safe reading. It is your expensive refrigerated and frozen inventory, after all.
Three real catches, all from our published case studies:
- The coldest point creeping up, chart 5. A defrost timer was failing in a Philadelphia walk-in freezer. TemperatureWise flagged a likely fault on the evaporator side 53 hours before the box peaked. One technician visit fixed it, and none of the $150,000 of frozen stock was lost. The freezer case study.
- The whole day running warmer, chart 6. The same 2,000 sq ft freezer, that summer, had a refrigerant leak on one system and a broken fan on the other. The first warning came nearly 30 days before repairs began, in time to move $450,000 of frozen stock into a temporary reefer. The dual-system case study.
- A straight climb, chart 1. A parked reefer stopped at noon on December 26 and could not restart on a weak battery. The alert went out at 4 PM, 5 hours 45 minutes before the box crossed its safety line. The reefer case study.
TemperatureWise does not replace your technician. It tells you which one to call, and when. See how TemperatureWise works. Why a fixed alarm line catches only the end of most of these shapes is on threshold vs rate-of-change alerting.
Some faults show to your eyes before they show in the readings
A shape in the readings is a pointer, not a diagnosis. Here is what temperature readings alone cannot do:
- Show water, fog and frost. 7 of the 51 symptoms on our troubleshooting pages leave no shape in the air temperature at first. Water on the floor, glass doors that fog, a cabinet that sweats and frost thickening on a prep rail's walls all show to your eyes first. TemperatureWise sensors read humidity too, which can show a failing door seal or an icing coil before the temperature moves.
- Catch short cycling. A compressor that clicks on and off every few minutes is mostly too fast for the readings to catch. You hear it first. Time a few cycles with your phone and tell the technician.
- Tell apart shapes that look alike. A dirty condenser and a leaking door gasket both make a box slip on busy afternoons. A slow leak and a worn compressor both make the whole day run warmer. That is why TemperatureWise names the top three likely parts, not one, and why a technician confirms the fault.
- See past one spot. A shape that needs two sensors, such as one end of a reefer warming while the other holds, shows only where a box has two.