Carel AL23

What does Carel AL23 mean?

AL23 on Carel µChiller and pCO chiller controllers signals a fault with probe B2. B2 is commonly the chilled water outlet or return temperature probe on chiller installations. When B2 fails, the temperature differential calculation across the heat exchanger is unavailable, and any control or alarm function relying on B2 — including process temperature monitoring — is disabled.

Symptoms

• AL23 displayed on the Carel controller.
• Chilled water outlet or return temperature monitoring is unavailable.
• Control functions dependent on B2 are disabled or running in fallback mode.
• Manual reset required after probe repair.

Common causes

• B2 probe wiring broken or disconnected at the controller or probe end.
• NTC sensor element failed.
• Probe immersion well corroded or blocked, pulling the probe out of thermal contact.
• Probe cable pinched or damaged by pipework movement.

Diagnostic steps

1. 01

   Locate and identify the B2 probe

   Confirm B2's physical location from the chiller wiring diagram. It is often an immersion probe in the water outlet pipe or a surface-mounted probe on the heat exchanger.

2. 02

   Inspect B2 probe wiring

   Check the B2 terminal at the controller and the probe connector. Look for damaged insulation, corroded pins, or loose screws.

3. 03

   Measure B2 resistance

   Disconnect the probe and measure resistance with a multimeter. Compare against the NTC characteristic table — at 20 °C, expect approximately 10 kΩ.

4. 04

   Replace the probe

   Install a Carel-compatible NTC probe in the same immersion well or mounting location. Ensure the probe tip has good thermal contact.

5. 05

   Reset AL23 and verify readings

   After fitting the replacement probe, reset the alarm and confirm B2 reads a plausible water temperature that aligns with B1 readings before restarting the unit.

When to call a professional

B2 immersion probe replacement in a pressurized chilled water circuit requires circuit isolation, draining, and re-pressurization, which must be performed by a qualified engineer. Escalate to a Carel-authorized technician if AL23 persists after probe replacement, as this indicates a controller analog input fault requiring board-level diagnosis.