Carel AL020

What does Carel AL020 mean?

AL020 is an external humidity probe alarm on the µChiller system. The controller has detected that the analog signal from the external ambient humidity probe is out of its configured valid range — either a disconnected probe wire, a short circuit, or a failed sensor element. Control logic that depends on humidity input is degraded until the probe is restored.

Symptoms

• AL020 flashes on the controller display with buzzer activation.
• External humidity reading shows an invalid or frozen value.
• Functions using the humidity input (e.g., drycooler control, free cooling compensation) may default to fallback settings.
• Alarm relay activates depending on controller configuration.

Common causes

• Humidity probe cable disconnected or broken at the terminal block.
• Short circuit in the probe wiring to the controller analog input.
• Probe sensor element failed due to age, moisture ingress, or damage.
• Incorrect probe type configured in parameters not matching installed hardware.
• Probe supply voltage absent or outside tolerance.

Diagnostic steps

1. 01

   Inspect probe wiring at the terminal block

   Power down and check the signal and supply wires at the controller input terminal. Tighten any loose connections and look for chafed or broken conductors.

2. 02

   Measure probe supply voltage

   With power on, measure the 24 V supply to the probe. No voltage indicates a blown fuse or wiring fault upstream of the probe.

3. 03

   Check probe output signal

   Measure the analog output of the probe (typically 4–20 mA or 0–10 V). A reading of 0 mA or 0 V when supply is present usually indicates a failed sensor or open circuit.

4. 04

   Verify probe configuration parameters

   In the service programming level, confirm the probe type and input range match the physical sensor installed. A mismatch causes the controller to reject in-range signals.

5. 05

   Replace the probe if defective

   If wiring and configuration are correct but the signal is still invalid, replace the humidity probe with a Carel-approved equivalent and recalibrate if required.

When to call a professional

Engage a qualified technician if replacing the probe does not resolve the alarm, or if the analog input card itself is suspected. Input module replacement and system reconfiguration require Carel parameter-level access and calibration tools to restore correct dew-point compensation and free-cooling control.