Carel AL025 Error Code

What does Carel AL025 mean?

AL025 is a fault alarm for the LT (Low Temperature) chiller inlet water temperature probe. The NTC or PT1000 sensor measuring the incoming water temperature on the LT circuit has failed, is disconnected, or is reading outside the configured alarm limits. The controller immediately shuts down the LT circuit and logs the event.

Symptoms

• LT circuit shuts down immediately on alarm activation.
• AL025 shown on display with buzzer active.
• LT chiller inlet water temperature reads an invalid value or maximum/minimum limit.
• Free cooling or heat pump functions depending on this probe are suspended.

Common causes

• NTC probe wire disconnected or broken at the controller input terminal.
• Probe sensor element failed — reads open circuit or short circuit.
• Probe installed in a location causing damage from water hammer or vibration.
• Incorrect probe calibration offset pushing a marginal reading outside alarm limits.
• Probe type in configuration does not match the physical probe installed.

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Diagnostic steps

1. Inspect probe wiring at the controller input

   Check the NTC probe wires at the input terminal. An open circuit reads very high resistance; a short circuit reads near zero. Either condition triggers the alarm.

2. Measure probe resistance

   Disconnect the probe from the terminal and measure its resistance with a multimeter. Compare the reading to the NTC resistance-temperature table for the probe type. A value far outside the expected range confirms sensor failure.

3. Check probe installation condition

   Inspect the probe well or immersion fitting for physical damage or corrosion. A damaged probe sleeve allows water ingress, which can short the sensor element.

4. Review probe configuration parameters

   In the service menu, verify the probe type and alarm limit parameters. An offset that shifts the measured value outside the alarm window causes false alarms on an otherwise functional probe.

5. Replace the probe and verify operation

   Install a compatible replacement probe with a matching temperature range and connector type. Confirm the LT inlet temperature reading is plausible before restarting the circuit.

When to call a professional

A technician should be involved if the probe alarm coincides with actual abnormal water temperatures or if replacing the probe does not clear the fault, which may indicate a faulty analog input card. Water-side commissioning, probe calibration, and parameter configuration beyond the user level require Carel service access credentials.