Carel AL023

What does Carel AL023 mean?

AL023 signals a fault on the MT (Medium Temperature) circuit inlet pressure probe. The 4–20 mA or ratiometric pressure transducer at the MT circuit inlet has produced a reading outside the configured valid range, indicating either a wiring fault, failed transducer, or probe input out of range. The affected circuit shuts down as a serious alarm.

Symptoms

• MT circuit 1 shuts down immediately on alarm activation.
• AL023 displayed on the controller with the spanner icon if technical service is flagged.
• MT inlet pressure reading shows an invalid or static value on the display.
• Alarm relay activates based on relay configuration.

Common causes

• Pressure probe connector is loose or disconnected at the controller terminal.
• Transducer has failed internally due to overpressure, moisture, or age.
• Wiring short circuit causing signal to read at the bottom of the range.
• Incorrect probe type selected in configuration parameters.
• Refrigerant system pressure far outside the probe measurement range.

Diagnostic steps

1. 01

   Inspect probe wiring at controller and transducer

   Check signal and power wires at both the controller terminal block and the transducer body. Tighten any loose connections and verify the connector is fully seated.

2. 02

   Measure probe supply voltage

   With power on, confirm the transducer receives its rated supply voltage (typically 5 V or 24 V). Absence of supply voltage points to a wiring break or blown fuse.

3. 03

   Check transducer output signal

   Measure the analog output of the probe with a meter. An out-of-range signal — such as 0 mA on a 4–20 mA device with known system pressure — confirms transducer failure.

4. 04

   Verify probe configuration parameters

   In the service menu, confirm the probe type and pressure range match the installed transducer specifications. An incorrect range setting causes valid signals to appear as alarm conditions.

5. 05

   Replace the transducer if defective

   Recover refrigerant if required by local regulations, replace the failed transducer with the correct Carel or OEM specified model, re-pressurize, and leak-check before restarting.

When to call a professional

Refrigerant recovery and recharge required for transducer replacement must be performed by an EPA 608-certified (or equivalent) technician. Call a pro if the circuit stays in alarm after probe replacement, which may indicate refrigerant charge issues or actual abnormal pressures requiring diagnostic evaluation beyond probe checks.