The thermostat or temperature controller reads the box temperature and switches cooling on and off, and in many boxes manages defrost and alarms. Signs of failure include readings that do not match a thermometer, no cooling, constant running or wide swings. Faulty sensors are a common cause.
Modern walk-ins often have an electronic controller with a display and menus. Older ones use a simple dial thermostat. Either way, the box can only be as accurate as the sensor and the settings.
What it does
The controller compares the sensor reading with the setpoint and a differential. When temperature rises above setpoint plus differential, it starts cooling. When it drops to setpoint, it stops. Electronic models also control defrost timing, fan operation and alarms. Wiring or setting errors can cause behavior that looks like an equipment fault.
Signs it is failing
- Display disagrees with an independent thermometer.
- Box swings widely above and below the setpoint.
- Unit runs constantly or hardly at all.
- Blank or flickering display.
- Alarms that do not match the real temperature.
- Settings changed after an outage.
Common causes of failure
| Cause | What happens |
|---|---|
| Faulty sensor | Wrong readings feed the wrong decisions |
| Wrong settings | Setpoint, differential or defrost values changed |
| Power problems | Surges damage the board |
| Moisture | Water in the enclosure |
| Wiring fault | Sensor or relay connections loose |
What you can safely check
- Compare readings. Use an independent thermometer in a jug of water at box temperature.
- Note the settings. Photograph the display and values.
- Check for alarms. Write down any codes.
- Do not reset to defaults. This may erase important settings.
This part is on live power. Do not open the electrical box, bridge terminals or reset repeatedly. Only a qualified technician should test or replace it.
What a technician does
- Checks sensor accuracy and placement.
- Reviews and restores correct parameters.
- Tests relays and outputs.
- Repairs wiring and seals the enclosure.
- Replaces the controller or sensor if needed.
Keeping it healthy
- Record settings and keep them on file.
- Check accuracy with an independent thermometer.
- Protect the controller from moisture and surges.
- Change batteries in wireless alarms on schedule.
A controller reading that looks fine is not proof the box is at temperature. The independent check is what protects your food.
When to call for service
Call if the display is blank, the box will not hold temperature or alarms sound without cause. Before you call, check with an independent thermometer and photograph the settings. If food temperature is above 41 degrees, act on the food first. Keep a record of when the display last looked normal.
Repair or replace
Sensors are inexpensive and are replaced first when readings are wrong. A failed controller board is replaced as a unit. Settings problems are corrected without parts. See temperature swings and temperature sensors.
Related reading: the parts and components guide, temperature logs and the troubleshooting guide.
Questions about this part
What setpoint should I use?
Set it so food stays at 41 degrees or below, with a margin. Ask your technician.
Why does the display show a different number from my thermometer?
A sensor may be misplaced or drifting. Check both against a reference.
Can I reset the controller to factory defaults?
Only with guidance. Defaults may not match your system.