Carel AL003

What does Carel AL003 mean?

AL003 on the Carel FLSTDmSCHE screw chiller controller means probe U3 is broken or disconnected. Probe U3 is one of the unit-level monitoring sensors (such as outdoor ambient temperature, entering water temperature, or another system parameter depending on chiller configuration). Unlike AL002, AL003 triggers no shutdown action — it is a signal-only alarm with automatic reset, meaning the unit continues running but the affected measurement is unavailable.

Symptoms

• AL003 displayed on the pCO5 terminal; unit continues running
• Probe U3 reading absent or out-of-range on the controller display
• Functions dependent on probe U3 data may revert to default or fixed values

Common causes

• Probe U3 wiring break or loose terminal at the pCO5 analogue input
• Probe U3 sensor failure with output outside the valid measurement range
• Environmental damage to the U3 probe — physical impact, moisture, or UV degradation if outdoors
• Incorrect probe type configured in the controller parameters for the U3 input

Diagnostic steps

1. 01

   Identify probe U3 function and location

   Consult the chiller wiring diagram to determine what U3 measures on this specific installation. Outdoor ambient and entering water temperature probes are common U3 assignments.

2. 02

   Inspect probe cable and terminals

   With the unit running (AL003 does not require shutdown), inspect the U3 probe cable for damage and verify terminal screws are tight at both the probe connector and the pCO5 input terminal.

3. 03

   Measure probe signal

   Measure the probe output at the pCO5 analogue input terminal. Confirm the reading matches the expected value for the current ambient or water temperature conditions.

4. 04

   Replace probe if faulty

   If the probe signal is absent or incorrect, replace U3 with a compatible CAREL sensor. Match the probe type (NTC, PT1000, or active) to the configuration set in the controller parameter for that input.

5. 05

   Confirm automatic alarm reset

   AL003 resets automatically 10 seconds after the probe signal returns to the valid range. Verify the alarm clears and the probe reading on the controller display is plausible.

When to call a professional

While AL003 does not shut down the chiller, running without a valid probe U3 reading can degrade performance optimization functions like ambient temperature setpoint reset. If probe replacement does not clear the alarm, a Carel-certified technician should verify pCO5 input board integrity.