Carel AL002

What does Carel AL002 mean?

AL002 on the Carel FLSTDmSCHE screw chiller controller indicates that probe U2 — one of the main unit temperature or pressure monitoring sensors — is broken or disconnected. When AL002 fires, the controller executes a controlled shutdown of all compressors and takes the unit offline to prevent operation without critical sensor feedback. The alarm resets automatically once the probe signal returns to a valid range.

Symptoms

• AL002 displayed on the pCO5 terminal with the entire unit shut down in controlled compressor sequence
• Probe U2 reading absent or displaying an out-of-range value on the controller display
• All cooling output lost until the probe fault is cleared

Common causes

• Probe U2 wiring break or loose terminal at the pCO5 analogue input
• Probe U2 sensor failure — resistance or signal output outside the controller's valid measurement range
• Connector corrosion or moisture ingress at the U2 probe terminal block
• Probe U2 physically damaged — crushed, cracked, or displaced from its measurement point

Diagnostic steps

1. 01

   Identify probe U2 location

   Refer to the chiller wiring diagram to locate the physical position of probe U2 on the unit. U2 is typically a return water temperature, evaporator outlet, or unit ambient sensor depending on the chiller configuration.

2. 02

   Inspect probe wiring and connection

   Shut down the unit and inspect the U2 probe cable from the sensor to the pCO5 analogue input terminal. Look for broken wires, loose screw terminals, and connector corrosion.

3. 03

   Measure probe output

   Reconnect power and measure the probe signal at the pCO5 board terminal with a multimeter. For NTC probes, check resistance values (approximately 10 kΩ at 25°C). For 4–20 mA probes, verify the current output is within range.

4. 04

   Replace probe if defective

   If measurement confirms sensor failure, replace probe U2 with a CAREL-compatible sensor of the correct type and range. Ensure the replacement probe is securely mounted at the original measurement location.

5. 05

   Verify automatic reset and unit restart

   After probe replacement, AL002 should reset automatically once the signal returns to the valid range (10-second delay per the alarm table). Confirm the unit restarts and all compressors return to normal operation.

When to call a professional

If the unit does not restart after probe replacement and alarm reset, or if AL002 recurs within the same operating session, a Carel-certified technician should inspect the pCO5 analogue input board. For probes measuring refrigerant-circuit parameters (evaporator outlet), any probe replacement work near refrigerant lines requires EPA 608 certification.