Carel AL035

What does Carel AL035 mean?

AL035 signals that a serial humidity probe on the µChiller network is offline or has broken communication. The controller expected data from a connected serial humidity sensor but did not receive a valid response within the timeout period. Humidity-dependent control functions such as free cooling enable/disable are suspended.

Symptoms

• AL035 displayed on controller with alarm notification.
• Serial humidity probe reading shows invalid or last-good frozen value.
• Humidity-dependent control modes revert to fallback or default behavior.
• Alarm relay activates per configured relay output assignment.

Common causes

• Serial bus cable disconnected or broken between the controller and humidity probe.
• Incorrect serial address configured on the probe or in controller parameters.
• Humidity probe powered off or failed internally.
• Serial communication interference from nearby electrical equipment.
• Probe firmware version incompatible with the controller.

Diagnostic steps

1. 01

   Inspect serial bus wiring to the probe

   Check the RS-485 or pLAN bus cable connecting the humidity probe. Verify polarity (A/B) is correct and that both ends are securely terminated at the controller and probe.

2. 02

   Verify probe serial address

   Confirm the probe's configured serial address matches the address assigned to it in the controller's serial device table. Address conflicts with other devices cause communication failures.

3. 03

   Check probe power supply

   Measure the supply voltage at the probe terminals. If absent, trace the supply circuit for a blown fuse or open wire.

4. 04

   Check for serial bus interference

   Ensure the serial cable is routed away from power cables and inverter drives. Add termination resistors at cable ends if the bus run exceeds the recommended maximum length.

5. 05

   Replace the probe if diagnostics fail

   If wiring, power, and addressing are all confirmed correct, replace the humidity probe. Use a Carel-compatible serial humidity sensor with matching protocol and firmware.

When to call a professional

Involve a qualified Carel technician if the serial bus configuration involves multiple networked devices that are difficult to diagnose in the field, or if the alarm occurs alongside other pLAN communication faults suggesting a bus-level issue. Reprogramming serial device tables at the manufacturer level requires Carel access credentials.