Kolpak Walk-In Display SF Alternating with OP: Open Sensor
On a Kolpak Polar-Pak system with the A421 control, a display that alternates between SF and OP means the temperature sensor or its wiring is open. Note the pattern, check the walk-in temperature with a thermometer and cycle the power once at the disconnect. If the pattern comes back, call us.
Medium: cooling may continue, but not controlled by the real box temperature.
- Code
- SF alternating with OP
- Maker
- Kolpak
- Equipment
- Walk-in coolers & freezers
- Applies to
- Johnson Controls A421 electronic temperature control (Polar-Pak drop-in systems)
What does Kolpak SF alternating with OP mean?
The A421 reads the box temperature through a sensor on a plugged wire. If the sensor, its plug or its wire opens, the control loses its box reading. It then flashes OP for the open circuit and SF for the sensor failure, switching between them on the display. With no reading, the control keeps the output working according to the selected SF mode, so the walk-in may keep cooling, but not by its true temperature. This applies to Polar-Pak drop-in systems such as the PCA050BT2, PCE075BT3 and PFE100BT3. The pattern stays on the display until the open circuit is repaired. Because the box could run too warm or too cold, a thermometer check is the only way to know what is happening in the box, and a restart does not give the control a sensor reading back.
Affected models and control systems
Kolpak Polar-Pak drop-in systems with the Johnson Controls A421 electronic temperature control, such as the PCA050BT2, PCE075BT3 and PFE100BT3. Other controls use different displays.
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
- Display alternating between SF and OP
- Box temperature different from the setpoint
- Compressor running on a fixed pattern instead of by temperature
- No normal temperature readout
Possible causes
Listed in Kolpak's documentation
- Open temperature sensor or sensor wiring
- Control running in its selected SF mode
Other likely causes — what usually sits behind SF alternating with OP on this control. A technician confirms which one it is before any part is replaced.
- The temperature sensor and its plug
- Sensor wiring between the box and the control
- The sensor input on the A421 control
- The SF mode selected in the control
Safe checks before calling for service
- Read the display and note that it alternates between SF and OP.
- Check the walk-in temperature with a thermometer.
- The owner's manual says to cycle the power to reset the control, so switch the system off at its disconnect, wait, and switch it on once.
- If the pattern returns, call us and tell us what the thermometer shows.
How a technician diagnoses SF alternating with OP
- Confirm the SF and OP alternation and the SF mode setting.
- Check the walk-in temperature with a reference thermometer.
- Inspect the sensor plug and wire run for a break or corrosion.
- Measure the sensor resistance against a reference temperature.
- Test the A421 sensor input, and replace the sensor, wire or control as needed.
- The sensor wire runs between the box and the control cabinet, near line-voltage compressor circuits. Interpreting the SF mode and resistance readings needs the control's tables. The box can drift to an unsafe temperature unnoticed.
How SF alternating with OP differs from similar codes
This differs from SF alternating with SH, which means the sensor circuit is shorted rather than open.
Technical reference
- Document
- POLAR-PAK Drop-In Refrigeration System IOM, P/N 550002878-14 (4/25), Troubleshooting table (Johnson Controls A421)
- Edition
- Part Number: 550002878-14 4/25
- Models / control
- Johnson Controls A421 electronic temperature control (Polar-Pak drop-in systems)
- Section
- Troubleshooting - A421 fault code table, p. 19
- 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 Kolpak walk-in cooler & freezer error codes
Is the walk-in still cooling?
It may be, but not by its true temperature, so check with a thermometer.
Will cycling the power fix it?
The manual calls for it, but an open sensor circuit usually returns.
Is the sensor expensive?
The sensor is generally a small part; the cost is mostly in tracing the open.
Need help with Kolpak SF alternating with OP?
A technician confirms the cause on site and gives you the price in writing before any work starts.