Polar King Walk-In Cooler Alarm T4 AUX SENSOR: Shorted or Open
T4 AUX SENSOR is an alarm on the KE2 evaporator control: the auxiliary temperature sensor is shorted or open. The document gives no further detail, so check whether the walk-in is holding temperature and clear the alarm once to see if it returns.
Low: an auxiliary input, and the document describes no effect on cooling.
- Code
- T4 AUX SENSOR
- Maker
- Polar King
- Equipment
- Walk-in coolers & freezers
- Applies to
- Polar King walk-in coolers/freezers with KE2 Evaporator Efficiency Evap OEM board (Basic Display / Combo Display)
What does Polar King T4 AUX SENSOR mean?
Besides the suction, return air and coil sensors, the KE2 Evaporator Efficiency board has a fourth temperature input that it calls the auxiliary sensor, T4. When its wiring is shorted or its circuit is open, the board records T4 AUX SENSOR in the Alarms Status Menu. The Combo Display shows the words; the Basic Display shows a red or yellow light. The installation instructions state the trigger and nothing more, and the CLEAR ALARMS menu item is the only reset mentioned. They do not say what the auxiliary sensor is used for in a given installation, so we cannot tell you what is affected. An auxiliary sensor that is not installed at all is a different situation, and the document does not discuss it either, so any statement about that would be a guess. The practical step is to learn from the installer or technician which sensor is wired to T4 on your box.
Affected models and control systems
Polar King walk-in coolers and freezers fitted with the KE2 Evaporator Efficiency Evap OEM board, with either the Basic Display or the Combo Display. The alarm is listed in the Alarms Status Menu, which is view only, on page 14 of the KE2 installation instructions.
Code numbers and their meaning can change between control generations. We match the full model and control before acting on a code.
What you may notice
- Red or yellow alarm light
- T4 AUX SENSOR on the Combo Display
- No obvious change in box temperature
Possible causes
Likely causes — what usually sits behind T4 AUX SENSOR on this control. A technician confirms which one it is before any part is replaced.
- An auxiliary sensor lead that was cut or unplugged during other work on the box
- A connector at the board corroded in a damp room
- A sensor that was fitted for a purpose no longer in use
Safe checks before calling for service
- Note the alarm and check the box temperature with a thermometer.
- Clear the alarm with CLEAR ALARMS and see whether it returns.
- If it returns, ask whoever installed the control what T4 is connected to.
- Record the answer, the time and the temperature for the service call.
How a technician diagnoses T4 AUX SENSOR
- Read the alarm and the sensor configuration on the board.
- Establish what T4 is wired to on this installation.
- Check the lead and connector for damage or corrosion.
- Measure sensor resistance and compare it with the chart.
- Replace the sensor if it reads open or shorted, or correct the configuration if it is unused.
- Identifying what T4 serves needs the installation record and the board's configuration, which an operator does not have. Testing the sensor means disconnecting a board with live control wiring. Changing the configuration to hide the alarm can remove a protection the box was set up with.
How T4 AUX SENSOR differs from similar codes
T4 is the auxiliary sensor. Unlike T1 to T3, the document does not tie it to a particular place on the evaporator.
Technical reference
- Document
- KE2 Evaporator Efficiency Installation Instructions N.1.1 (posted on polarking.com Downloads as ke2-installation.pdf; authored by KE2 Therm Solutions)
- Edition
- N.1.1 July 2015
- Models / control
- Polar King walk-in coolers/freezers with KE2 Evaporator Efficiency Evap OEM board (Basic Display / Combo Display)
- Section
- Alarms Status Menu (view only), p. 14 (Alarm Status Menu listed p. 13)
- Checked
- October 2026
We use the maker's document to read the code and do not reproduce it. Always confirm the complete model and control before applying these steps.
Related Polar King walk-in cooler & freezer error codes
What does T4 measure?
The document calls it the auxiliary sensor and does not say where it is mounted.
Is the walk-in affected?
The document does not say. Check the box temperature directly.
Can I ignore T4 AUX SENSOR?
If the sensor is not in use, a technician may be able to confirm that, but do not assume it.
Need help with Polar King T4 AUX SENSOR?
A technician confirms the cause on site and gives you the price in writing before any work starts.