Carel AL03

What does Carel AL03 mean?

AL03 on Carel µChiller and pCO-based chiller controllers indicates that the high-pressure (HP) safety switch on compressor 2's refrigerant circuit has tripped. When refrigerant discharge pressure exceeds the switch's cutout setting, the contact opens and the controller shuts down compressor 2 circuit 1 with a manual-reset alarm. This is a critical safety protection to prevent compressor damage from over-pressure.

Symptoms

• AL03 displayed on the Carel controller.
• Compressor 2 is shut down; circuit 1 is off.
• High discharge pressure present or recently present on circuit 1.
• Manual reset required at the controller after the fault is resolved.

Common causes

• Condenser coils fouled or airflow blocked, preventing adequate heat rejection.
• Condenser fan motor failed, stopping airflow across the coil.
• Non-condensable gases (air) in the refrigerant circuit raising discharge pressure.
• Refrigerant overcharge causing elevated condensing pressure.
• High-pressure switch set point too low or device malfunctioning.

Diagnostic steps

1. 01

   Identify the elevated pressure condition

   Connect refrigerant gauges to compressor 2's high-side service port and record discharge pressure to confirm an actual over-pressure event.

2. 02

   Inspect the condenser section for compressor 2

   Check condenser coils and fans serving circuit 1. Clean fouled coils and verify all condenser fans are rotating at correct speed.

3. 03

   Verify condenser airflow is unobstructed

   Confirm the unit is not installed in an enclosure with recirculated discharge air. Ensure 1 m minimum clearance on all air intake and discharge sides.

4. 04

   Test the HP pressure switch

   If discharge pressure is within normal range but AL03 tripped, the HP switch may be faulty. Test continuity across the switch contacts with pressure at a known normal value.

5. 05

   Reset the alarm at the controller

   After correcting the root cause, reset AL03 manually at the controller and verify compressor 2 starts and runs without recurrence.

When to call a professional

Call a refrigeration engineer for all tasks involving refrigerant circuit pressure: attaching gauges, checking for non-condensables, or recovering and recharging refrigerant. HP switch replacement on a live system also requires lock-out/tag-out procedures that should only be performed by qualified personnel. Repeated HP trips without an obvious condenser fault suggest internal refrigerant system issues requiring thorough investigation.