Carel AL73

What does Carel AL73 mean?

AL73 indicates that the S3 probe — the discharge pressure sensor wired to Driver 1 of the EVD electronic expansion valve controller — is reading out of range or has lost continuity. On air-conditioning units using the Carel Standard Air-Conditioners program, this probe monitors discharge pressure for Compressor 1. When AL73 fires, the controller executes a controlled compressor shutdown to protect the refrigerant circuit.

Symptoms

• AL73 displayed on the pCO terminal with Compressor 1 shut down in a controlled sequence
• Discharge pressure reading absent or stuck at an implausible value on the controller display
• Unit enters alarm state; cooling output stops for the affected circuit

Common causes

• Wiring break or loose terminal on the S3 probe cable between the pressure transducer and the EVD driver
• Pressure transducer failure — probe output outside the 4–20 mA or ratiometric range expected by the driver
• Connector corrosion or moisture ingress at the probe terminal block on the EVD driver board
• Incorrect probe type configured in the driver parameters

Diagnostic steps

1. 01

   Check probe wiring continuity

   With the unit off, inspect the S3 probe cable from the discharge pressure transducer back to the EVD Driver 1 terminal block. Look for pinched, corroded, or disconnected wires.

2. 02

   Verify terminal tightness

   Re-seat the S3 signal and power wires on the EVD driver connector. Terminals can loosen due to vibration; retighten to the torque spec in the EVD installation manual.

3. 03

   Measure transducer output

   With the unit powered, use a multimeter to confirm the S3 transducer is producing the correct signal (e.g., 4–20 mA proportional to system pressure). A reading at or below 4 mA with the compressor off may indicate a dead transducer or open circuit.

4. 04

   Verify EVD parameter configuration

   Access the EVD Driver 1 service-level parameters and confirm the Probe S3 type matches the installed transducer. A mismatch causes the driver to interpret valid signals as faults.

5. 05

   Replace transducer if faulty

   If the transducer output is absent or erratic under all conditions, replace it with a CAREL-compatible unit of the same range and signal type. Recalibrate the probe alarm thresholds after replacement.

When to call a professional

Engage a certified refrigeration technician if the probe replacement does not clear AL73, or if the discharge pressure readings after repair indicate an underlying high-side pressure issue. Working on the discharge side of a live refrigerant circuit requires proper recovery equipment and EPA 608 certification. EVD driver EEPROM or board faults that co-occur with this alarm also require factory-level diagnostics.