Carel AL09

What does Carel AL09 mean?

AL09 on Carel µChiller and pCO chiller controllers indicates that the low-pressure (LP) safety switch on compressor 2's cooling circuit has tripped. When suction pressure drops below the LP switch cutout point, the contact opens and compressor 2 is shut down. This protects the compressor from operating in a vapor-starved condition that would result in overheating and mechanical failure.

Symptoms

• AL09 displayed on the Carel controller.
• Compressor 2 is shut down by the LP protection.
• Suction pressure on compressor 2's circuit is at or below the LP switch cutout pressure.
• Manual reset required after identifying and correcting the fault.

Common causes

• Refrigerant undercharge or leak causing insufficient suction pressure.
• Evaporator coil heavily iced over, blocking airflow and reducing evaporating pressure.
• Expansion valve stuck closed or thermostatic bulb charge lost.
• Suction line filter/dryer blocked, creating excessive pressure drop.
• LP pressure switch set too high relative to actual low-load operating conditions.

Diagnostic steps

1. 01

   Measure suction pressure with gauges

   Connect refrigerant gauges to compressor 2's suction service port. Confirm actual suction pressure is at or below the LP cutout setting.

2. 02

   Inspect the evaporator for ice buildup

   Check the evaporator coil for icing. If iced over, initiate a manual defrost and check why defrost is not functioning correctly.

3. 03

   Check for refrigerant loss

   Use an electronic leak detector to scan all refrigerant joints, valve stems, and brazed connections on compressor 2's circuit for leaks.

4. 04

   Check expansion valve operation

   Verify the thermal expansion valve (or EEV) bulb is securely clamped to the suction line and insulated. An unclipped or failed bulb causes the valve to close fully, dropping suction pressure.

5. 05

   Reset the alarm after correction

   After addressing the root cause, reset AL09 at the controller and monitor suction pressure for at least one full operating cycle before leaving the site.

When to call a professional

All refrigerant circuit work — leak detection with accurate electronic equipment, refrigerant recovery, recharge, and expansion valve replacement — requires F-Gas certification. Call a licensed refrigeration engineer. Do not reset and restart the compressor repeatedly without identifying the cause; operating a compressor with low suction pressure causes rapid oil and bearing damage.