Kolpak Walk-In Display SF Alternating with SH: Shorted Sensor

What does Kolpak SF alternating with SH mean?

The A421 trusts its sensor only while the reading stays in a believable range. A shorted sensor or a pinched wire produces a reading that cannot be right, and the control flashes SH for the short and SF for the sensor failure, taking turns on the display. Since the control has no true box temperature, it falls back on its SF mode for the output. The walk-in may keep running, but the control no longer follows the true box temperature. This applies to Polar-Pak drop-in systems such as the PCA050BT2, PCE075BT3 and PFE100BT3. A short is usually a physical problem: a crushed wire, a wet plug or a damaged sensor. The manual's reset is to cycle the power, which clears the display if the short was momentary. A thermometer is the only honest check of the box until the sensor is repaired.

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 SH
• Box temperature drifting from the setpoint
• Compressor running on a fixed pattern
• No normal temperature readout

Possible causes

Listed in Kolpak's documentation

• Shorted temperature sensor or sensor wiring
• Control running in its selected SF mode

Other likely causes — what usually sits behind SF alternating with SH on this control. A technician confirms which one it is before any part is replaced.

• The temperature sensor and its plug
• A pinched or damaged sensor wire
• The sensor input on the A421 control
• The SF mode setting

Safe checks before calling for service

How a technician diagnoses SF alternating with SH

1. Confirm the SF and SH alternation and the SF mode in use.
2. Verify the box temperature with a reference thermometer.
3. Inspect the sensor wire for crushed insulation or moisture at the plug.
4. Measure sensor resistance against a reference temperature.
5. Test the control input and replace the sensor, wire or control as needed.

How SF alternating with SH differs from similar codes

This differs from SF alternating with OP, which means the sensor circuit is open instead of shorted.

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.