Carel AL12

What does Carel AL12 mean?

AL12 on the Carel Standard Roof-top controller (program code +030221501, running on pCO1/pCO3/pCOxs hardware) indicates that the high-pressure safety switch on circuit 1 has opened. The controller immediately stops compressor 1 and locks circuit 1 off until a manual reset is performed. The alarm protects the compressor from operating at discharge pressures that would exceed its safe design limits.

Symptoms

• AL12 is displayed on the pCO terminal with circuit 1 compressor(s) stopped immediately.
• High-side (discharge) pressure on circuit 1 was at or above the high-pressure cutout setting when the switch opened.
• Manual alarm reset is required — the alarm does not clear automatically.
• Cooling capacity from circuit 1 is lost until the alarm is reset and the root cause is corrected.
• The alarm relay activates and the buzzer sounds.

Common causes

• Condenser fouled with dirt, debris, or grease reducing heat rejection and driving up discharge pressure.
• Condenser fans not running at full speed or stopped — motor failure, thermal overload trip, or inverter fault.
• Refrigerant overcharge causing excessive liquid volume and elevated condensing pressure.
• Outdoor ambient temperature temporarily very high combined with inadequate airflow around the unit.
• High-pressure cutout switch calibration has drifted, causing premature tripping below the design threshold.
• Blocked or partially closed discharge service valve restricting flow.

Diagnostic steps

1. 01

   Identify and correct the root cause before resetting

   Before pressing the alarm reset key, confirm that the high-pressure condition has resolved. Resetting into an uncleared fault will cause the compressor to trip again immediately and risks damaging the cutout switch from repeated cycling.

2. 02

   Inspect the condenser section

   Check all condenser fans are running and blowing air in the correct direction. Inspect coil surfaces for fouling. A blocked coil is the most common cause of high-pressure trips in warm weather — clean the coil from the inside out with compressed air or fin-cleaning solution.

3. 03

   Check discharge-side isolation valves

   Confirm that no service valves on the high-pressure side of circuit 1 are partially closed. A partially closed valve restricts refrigerant flow and causes rapid pressure build-up.

4. 04

   Verify the high-pressure cutout switch setting

   Using a manifold gauge set on circuit 1, confirm the actual discharge pressure when the switch opened. Compare to the factory cutout setting (typically 28–32 bar for HFC refrigerants). If the switch tripped well below its rated setting, the switch may need replacement.

5. 05

   Manually reset AL12 on the controller

   Once the root cause is resolved, press the Alarm button on the pCO terminal to display AL12, then press it again to reset. If the high-pressure condition has cleared, circuit 1 will restart after the configured minimum off-time has elapsed.

When to call a professional

Call a licensed refrigeration engineer if AL12 recurs after cleaning the condenser and confirming fans are operational, or if you suspect refrigerant overcharge, a defective high-pressure switch, or a refrigerant circuit restriction. Handling refrigerant — including checking charge or replacing components — requires F-Gas certification. A technician should also verify that the high-pressure cutout switch meets the system's safety requirements before leaving the unit in service.